diff --git a/CHANGELOG.md b/CHANGELOG.md
index 80b6fc7..9edc0df 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `data` field only, so the `meta.pagination` that would tell you the set was
   truncated is not part of what you get back.
 
+- `relationship_ids` on `Mappings.get()` / `get_iter()` and their async
+  counterparts, as a list or a comma-separated string. The server defaults to
+  `["Maps to"]`, so a composite concept returns only half its decomposition
+  unless `"Maps to value"` is asked for too: "Allergy to penicillin G" maps to
+  "Allergy to drug" via `Maps to` and to "penicillin G" via `Maps to value`.
+
 ### Fixed
 
 - `include_invalid=False` now reaches the server on `Mappings.get()` /
diff --git a/src/omophub/resources/mappings.py b/src/omophub/resources/mappings.py
index 5ce84ba..8e53e56 100644
--- a/src/omophub/resources/mappings.py
+++ b/src/omophub/resources/mappings.py
@@ -24,6 +24,7 @@ def get(
         concept_id: int,
         *,
         target_vocabulary: str | None = None,
+        relationship_ids: str | list[str] | None = None,
         include_invalid: bool | None = None,
         page: int = 1,
         page_size: int = 100,
@@ -40,6 +41,13 @@ def get(
         Args:
             concept_id: The concept ID
             target_vocabulary: Filter to a specific target vocabulary (e.g., "ICD10CM")
+            relationship_ids: Relationship types to return, as a list or a
+                comma-separated string. Defaults server-side to ``["Maps to"]``.
+                Pass ``["Maps to", "Maps to value"]`` to also get the
+                Value-as-Concept decomposition of composite concepts -- e.g.
+                "Allergy to penicillin G" maps to "Allergy to drug" via
+                ``Maps to`` and to "penicillin G" via ``Maps to value``, and the
+                default returns only the first of those.
             include_invalid: Whether to return mappings whose relationship or
                 target concept is deprecated. Omit to take the server default,
                 which for this endpoint is to **include** them; pass ``False``
@@ -59,6 +67,12 @@ def get(
         params: dict[str, Any] = {"page": page, "page_size": page_size}
         if target_vocabulary:
             params["target_vocabulary"] = target_vocabulary
+        if relationship_ids:
+            params["relationship_ids"] = (
+                ",".join(relationship_ids)
+                if isinstance(relationship_ids, list)
+                else relationship_ids
+            )
         if include_invalid is not None:
             params["include_invalid"] = "true" if include_invalid else "false"
         if vocab_release:
@@ -73,6 +87,7 @@ def get_iter(
         concept_id: int,
         *,
         target_vocabulary: str | None = None,
+        relationship_ids: str | list[str] | None = None,
         include_invalid: bool | None = None,
         page_size: int = 100,
         vocab_release: str | None = None,
@@ -86,6 +101,13 @@ def get_iter(
         Args:
             concept_id: The concept ID
             target_vocabulary: Filter to a specific target vocabulary (e.g., "ICD10CM")
+            relationship_ids: Relationship types to return, as a list or a
+                comma-separated string. Defaults server-side to ``["Maps to"]``.
+                Pass ``["Maps to", "Maps to value"]`` to also get the
+                Value-as-Concept decomposition of composite concepts -- e.g.
+                "Allergy to penicillin G" maps to "Allergy to drug" via
+                ``Maps to`` and to "penicillin G" via ``Maps to value``, and the
+                default returns only the first of those.
             include_invalid: Whether to return mappings whose relationship or
                 target concept is deprecated. Omit to take the server default,
                 which for this endpoint is to **include** them; pass ``False``
@@ -104,6 +126,12 @@ def fetch_page(
             params: dict[str, Any] = {"page": page, "page_size": size}
             if target_vocabulary:
                 params["target_vocabulary"] = target_vocabulary
+            if relationship_ids:
+                params["relationship_ids"] = (
+                    ",".join(relationship_ids)
+                    if isinstance(relationship_ids, list)
+                    else relationship_ids
+                )
             if include_invalid is not None:
                 params["include_invalid"] = "true" if include_invalid else "false"
             if vocab_release:
@@ -192,6 +220,7 @@ async def get(
         concept_id: int,
         *,
         target_vocabulary: str | None = None,
+        relationship_ids: str | list[str] | None = None,
         include_invalid: bool | None = None,
         page: int = 1,
         page_size: int = 100,
@@ -208,6 +237,13 @@ async def get(
         Args:
             concept_id: The concept ID
             target_vocabulary: Filter to a specific target vocabulary (e.g., "ICD10CM")
+            relationship_ids: Relationship types to return, as a list or a
+                comma-separated string. Defaults server-side to ``["Maps to"]``.
+                Pass ``["Maps to", "Maps to value"]`` to also get the
+                Value-as-Concept decomposition of composite concepts -- e.g.
+                "Allergy to penicillin G" maps to "Allergy to drug" via
+                ``Maps to`` and to "penicillin G" via ``Maps to value``, and the
+                default returns only the first of those.
             include_invalid: Whether to return mappings whose relationship or
                 target concept is deprecated. Omit to take the server default,
                 which for this endpoint is to **include** them; pass ``False``
@@ -227,6 +263,12 @@ async def get(
         params: dict[str, Any] = {"page": page, "page_size": page_size}
         if target_vocabulary:
             params["target_vocabulary"] = target_vocabulary
+        if relationship_ids:
+            params["relationship_ids"] = (
+                ",".join(relationship_ids)
+                if isinstance(relationship_ids, list)
+                else relationship_ids
+            )
         if include_invalid is not None:
             params["include_invalid"] = "true" if include_invalid else "false"
         if vocab_release:
@@ -241,6 +283,7 @@ async def get_iter(
         concept_id: int,
         *,
         target_vocabulary: str | None = None,
+        relationship_ids: str | list[str] | None = None,
         include_invalid: bool | None = None,
         page_size: int = 100,
         vocab_release: str | None = None,
@@ -254,6 +297,13 @@ async def get_iter(
         Args:
             concept_id: The concept ID
             target_vocabulary: Filter to a specific target vocabulary (e.g., "ICD10CM")
+            relationship_ids: Relationship types to return, as a list or a
+                comma-separated string. Defaults server-side to ``["Maps to"]``.
+                Pass ``["Maps to", "Maps to value"]`` to also get the
+                Value-as-Concept decomposition of composite concepts -- e.g.
+                "Allergy to penicillin G" maps to "Allergy to drug" via
+                ``Maps to`` and to "penicillin G" via ``Maps to value``, and the
+                default returns only the first of those.
             include_invalid: Whether to return mappings whose relationship or
                 target concept is deprecated. Omit to take the server default,
                 which for this endpoint is to **include** them; pass ``False``
@@ -272,6 +322,12 @@ async def fetch_page(
             params: dict[str, Any] = {"page": page, "page_size": size}
             if target_vocabulary:
                 params["target_vocabulary"] = target_vocabulary
+            if relationship_ids:
+                params["relationship_ids"] = (
+                    ",".join(relationship_ids)
+                    if isinstance(relationship_ids, list)
+                    else relationship_ids
+                )
             if include_invalid is not None:
                 params["include_invalid"] = "true" if include_invalid else "false"
             if vocab_release:
diff --git a/tests/unit/resources/test_mappings.py b/tests/unit/resources/test_mappings.py
index a4cefba..7b19131 100644
--- a/tests/unit/resources/test_mappings.py
+++ b/tests/unit/resources/test_mappings.py
@@ -59,6 +59,65 @@ def test_get_mappings_with_filters(
         assert "target_vocabulary=ICD10CM" in url_str
         assert "include_invalid=true" in url_str
 
+    @respx.mock
+    def test_get_mappings_sends_relationship_ids(
+        self, sync_client: OMOPHub, base_url: str
+    ) -> None:
+        """Value-as-Concept is unreachable without this parameter.
+
+        The server defaults to `Maps to` alone, so a composite concept returns
+        only half its decomposition unless `Maps to value` is asked for.
+        """
+        route = respx.get(f"{base_url}/concepts/4167462/mappings").mock(
+            return_value=Response(200, json={"success": True, "data": {"mappings": []}})
+        )
+
+        sync_client.mappings.get(4167462)
+        assert "relationship_ids" not in str(route.calls[0].request.url)
+
+        sync_client.mappings.get(4167462, relationship_ids=["Maps to", "Maps to value"])
+        assert route.calls[1].request.url.params["relationship_ids"] == (
+            "Maps to,Maps to value"
+        )
+
+        # A bare string is passed through unjoined, matching concepts.relationships().
+        sync_client.mappings.get(4167462, relationship_ids="Maps to value")
+        assert route.calls[2].request.url.params["relationship_ids"] == "Maps to value"
+
+    @respx.mock
+    def test_get_iter_forwards_relationship_ids(
+        self, sync_client: OMOPHub, base_url: str
+    ) -> None:
+        """The filter has to survive the pagination helper too."""
+        route = respx.get(f"{base_url}/concepts/4167462/mappings").mock(
+            return_value=Response(
+                200,
+                json={
+                    "success": True,
+                    "data": {"mappings": []},
+                    "meta": {
+                        "pagination": {
+                            "page": 1,
+                            "page_size": 100,
+                            "total_items": 0,
+                            "total_pages": 0,
+                            "has_next": False,
+                            "has_previous": False,
+                        }
+                    },
+                },
+            )
+        )
+
+        list(
+            sync_client.mappings.get_iter(
+                4167462, relationship_ids=["Maps to", "Maps to value"]
+            )
+        )
+        assert route.calls[0].request.url.params["relationship_ids"] == (
+            "Maps to,Maps to value"
+        )
+
     @respx.mock
     def test_get_mappings_include_invalid_is_tri_state(
         self, sync_client: OMOPHub, base_url: str