diff --git a/src/dicognito/release_notes.md b/src/dicognito/release_notes.md
index 35f4cc5..74b0ccd 100644
--- a/src/dicognito/release_notes.md
+++ b/src/dicognito/release_notes.md
@@ -2,6 +2,10 @@
 
 - Drop support for Python 3.10 ([#185](https://github.com/blairconrad/dicognito/issues/185))
 
+### Fixed
+
+- Anonymized UIs are not conformant DICOM UIDs. They now use the UUID-derived "2.25." form ([#189](https://github.com/blairconrad/dicognito/issues/189))
+
 ## 0.19.0
 
 ### Changed
diff --git a/src/dicognito/uianonymizer.py b/src/dicognito/uianonymizer.py
index 9406ea8..791c006 100644
--- a/src/dicognito/uianonymizer.py
+++ b/src/dicognito/uianonymizer.py
@@ -1,5 +1,6 @@
 """Replace UIs with a new value."""
 
+import uuid
 from collections.abc import Iterator
 
 import pydicom
@@ -8,6 +9,10 @@
 from dicognito.element_anonymizer import ElementAnonymizer
 from dicognito.randomizer import Randomizer
 
+# Fixed namespace for dicognito's name-based UUIDs. Never change it, or the same
+# seed will stop producing the same UIs.
+DICOGNITO_NAMESPACE = uuid.UUID("220d81d1-0385-470f-9194-f7c213aa7490")
+
 
 class UIAnonymizer(ElementAnonymizer):
     """
@@ -19,6 +24,7 @@ class UIAnonymizer(ElementAnonymizer):
     def __init__(self, randomizer: Randomizer) -> None:
         """Create a new UIAnonymizer."""
         self._randomizer = randomizer
+        self._seed_namespace = uuid.uuid5(DICOGNITO_NAMESPACE, randomizer.seed)
 
     def __call__(
         self,
@@ -62,4 +68,5 @@ def describe_actions(self) -> Iterator[str]:
         yield "Replace all UI elements with anonymized values"
 
     def _new_ui(self, ui: str) -> str:
-        return "2." + str(10**39 + self._randomizer.to_int(ui))
+        # A UID made from a UUID, as described in DICOM PS3.5 section B.2.
+        return f"2.25.{uuid.uuid5(self._seed_namespace, ui).int}"
diff --git a/tests/test_anonymize_single_instance.py b/tests/test_anonymize_single_instance.py
index cb3cf48..832e911 100644
--- a/tests/test_anonymize_single_instance.py
+++ b/tests/test_anonymize_single_instance.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import datetime as dt
+import uuid
 
 import pydicom
 import pytest
@@ -78,6 +79,58 @@ def test_repeated_identifying_uis_get_same_values(one_element_path, another_elem
         assert one_uid == another_uid
 
 
+@pytest.mark.parametrize(
+    "element_path",
+    [
+        "file_meta.MediaStorageSOPInstanceUID",
+        "SOPInstanceUID",
+        "StudyInstanceUID",
+        "SeriesInstanceUID",
+        "FrameOfReferenceUID",
+    ],
+)
+def test_identifying_uis_are_conformant_uuid_derived_uids(element_path):
+    with load_test_instance() as dataset:
+        anonymizer = Anonymizer()
+        anonymizer.anonymize(dataset)
+
+        actual = eval("dataset." + element_path)
+
+        assert_is_uuid5_derived_uid(actual)
+
+
+def test_multivalued_identifying_ui_gets_conformant_values():
+    with load_test_instance() as dataset:
+        original = ["1.2.3.1.1", "1.2.3.1.2"]
+        dataset.SourceImageSequence[0].ReferencedSOPInstanceUID = original
+
+        anonymizer = Anonymizer()
+        anonymizer.anonymize(dataset)
+
+        actual = dataset.SourceImageSequence[0].ReferencedSOPInstanceUID
+
+        assert len(set(actual)) == len(original)
+        for value in actual:
+            assert_is_uuid5_derived_uid(value)
+
+
+def test_identifying_uis_are_the_same_with_the_same_seed():
+    with load_test_instance() as dataset1, load_test_instance() as dataset2:
+        Anonymizer(seed="some seed").anonymize(dataset1)
+        Anonymizer(seed="some seed").anonymize(dataset2)
+
+        assert dataset1.SOPInstanceUID == dataset2.SOPInstanceUID
+        assert dataset1.StudyInstanceUID == dataset2.StudyInstanceUID
+
+
+def test_identifying_uis_differ_with_a_different_seed():
+    with load_test_instance() as dataset1, load_test_instance() as dataset2:
+        Anonymizer(seed="some seed").anonymize(dataset1)
+        Anonymizer(seed="another seed").anonymize(dataset2)
+
+        assert dataset1.SOPInstanceUID != dataset2.SOPInstanceUID
+
+
 @pytest.mark.parametrize(
     "element_path",
     [
@@ -733,3 +786,13 @@ def assert_element_is(dataset: pydicom.dataset.Dataset, element_name: str, expec
         assert element_name not in dataset
     else:
         assert expected == getattr(dataset, element_name)
+
+
+def assert_is_uuid5_derived_uid(uid: str) -> None:
+    assert uid.startswith("2.25.")
+    remainder = uid.removeprefix("2.25.")
+    assert remainder.isdigit()
+    assert remainder == "0" or not remainder.startswith("0")
+    derived_uuid = uuid.UUID(int=int(remainder))
+    assert derived_uuid.version == 5  # noqa: PLR2004
+    assert derived_uuid.variant == uuid.RFC_4122