diff --git a/pyproject.toml b/pyproject.toml
index 66a8bf9..2aa5c7d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -16,6 +16,7 @@ exclude = '''
 '''
 
 [tool.pytest.ini_options]
+xfail_strict=true
 filterwarnings = [
     "error",
     "ignore:SelectableGroups:DeprecationWarning",
diff --git a/src/dicognito/__main__.py b/src/dicognito/__main__.py
index 5aa176f..2dafeb7 100644
--- a/src/dicognito/__main__.py
+++ b/src/dicognito/__main__.py
@@ -130,10 +130,12 @@ def main(main_args: Optional[Sequence[str]] = None) -> None:
         help="Set the log level. May be one of DEBUG, INFO, WARNING, ERROR, or CRITICAL.",
     )
     parser.add_argument(
-        "--seed",  # currently only intended to make testing easier
-        help="The seed to use when generating random attribute values. Primarily "
-        "intended to make testing easier. Best anonymization practice is to omit "
-        "this value and let dicognito generate its own random seed.",
+        "--seed",
+        help="The seed to use when generating anonymized attribute values. "
+        "If the same value is supplied for subsequent dicognito invocations, then "
+        "the same input objects will result in consistent anonymized results. "
+        "Omitting this value allows dicognito to generate its own random seed, which "
+        "may be slightly more secure, but does not support reproducible anonymization.",
     )
     parser.add_argument("--version", action=VersionAction)
 
diff --git a/src/dicognito/anonymizer.py b/src/dicognito/anonymizer.py
index 78f2f32..1113636 100644
--- a/src/dicognito/anonymizer.py
+++ b/src/dicognito/anonymizer.py
@@ -52,9 +52,9 @@ class Anonymizer:
         id_suffix : str
             A prefix to add to all unstructured ID fields, such as Patient
             ID, Accession Number, etc.
-        seed
-            Not intended for general use. Seeds the data randomizer in order
-            to produce consistent results. Used for testing.
+        seed : Optional[str]
+            Seeds the data randomizer, which will produce consistent results when
+            invoked with the same seed.
         """
         minimum_offset_hours = 62 * 24
         maximum_offset_hours = 730 * 24
@@ -79,7 +79,7 @@ class Anonymizer:
                 "ReferencedPatientPhotoSequence",
                 "ResponsibleOrganization",
             ),
-            UIAnonymizer(),
+            UIAnonymizer(randomizer),
             PNAnonymizer(randomizer),
             IDAnonymizer(
                 randomizer,
diff --git a/src/dicognito/randomizer.py b/src/dicognito/randomizer.py
index aba519e..c81e676 100644
--- a/src/dicognito/randomizer.py
+++ b/src/dicognito/randomizer.py
@@ -11,9 +11,9 @@ class Randomizer:
         Parameters
         ----------
         seed
-            Not intended for general use. Seeds the data randomizer so it
-            produces consistent results when anonymizing elements with the
-            same initial values.
+            Used to convert input values into large integers.
+            The results are completely determined by the
+            given seed and the input value.
         """
         if seed is None:
             self.seed = str(os.urandom(20))
@@ -39,10 +39,10 @@ class Randomizer:
             result += c
         return result
 
-    def get_ints_from_ranges(self, original_value: Any, *suprenums: int) -> Sequence[int]:
+    def get_ints_from_ranges(self, original_value: Any, *suprema: int) -> Sequence[int]:
         """\
         Convert an original data element value into a series of
-        integers, each between 0 (inclusive) and one of the suprenums
+        integers, each between 0 (inclusive) and one of the suprema
         (exclusive) passed in.
 
         Parameters
@@ -55,7 +55,7 @@ class Randomizer:
         """
         big_int = self.to_int(original_value)
         result = []
-        for s in suprenums:
+        for s in suprema:
             result.append(big_int % s)
             big_int //= s
         return result
diff --git a/src/dicognito/release_notes.md b/src/dicognito/release_notes.md
index 88a7173..40af54d 100644
--- a/src/dicognito/release_notes.md
+++ b/src/dicognito/release_notes.md
@@ -2,6 +2,7 @@
 
 - Now assuming DA fields are 8 characters long ([#123](https://github.com/blairconrad/dicognito/issues/123))
 - Summary formatted as GitHub Flavored Markdown ([#125](https://github.com/blairconrad/dicognito/issues/125))
+- Anonymized UI values are now consistent between runs if the same seed is supplied ([#126](https://github.com/blairconrad/dicognito/issues/126))
 
 ## 0.13.0
 
diff --git a/src/dicognito/uianonymizer.py b/src/dicognito/uianonymizer.py
index c882e3f..0a05357 100644
--- a/src/dicognito/uianonymizer.py
+++ b/src/dicognito/uianonymizer.py
@@ -1,20 +1,15 @@
-from typing import Dict
-
-import collections
-import datetime
 import pydicom
 import pydicom.dataelem
-import random
+
+from dicognito.randomizer import Randomizer
 
 
 class UIAnonymizer:
-    def __init__(self) -> None:
+    def __init__(self, randomizer: Randomizer) -> None:
         """\
         Create a new UIAnonymizer.
         """
-        self._ui_map: Dict[str, str] = collections.defaultdict(self._new_ui)
-        self._creation_date: str = datetime.datetime.utcnow().strftime("%Y%m%d%H%M%S%f")
-        self._counter: int = 10000000
+        self._randomizer = randomizer
 
     def __call__(self, dataset: pydicom.dataset.Dataset, data_element: pydicom.DataElement) -> bool:
         """\
@@ -44,15 +39,10 @@ class UIAnonymizer:
             return False
 
         if isinstance(data_element.value, pydicom.multival.MultiValue):
-            data_element.value = list([self._ui_map[v] for v in data_element.value])
+            data_element.value = list(self._new_ui(v) for v in data_element.value)
         else:
-            data_element.value = self._ui_map[data_element.value]
+            data_element.value = self._new_ui(data_element.value)
         return True
 
-    def _new_ui(self) -> str:
-        self._counter += 1
-        counter_part = str(self._counter)
-        prefix = "2." + self._creation_date + "." + counter_part + "."
-        random_begin = pow(10, 63 - len(prefix))
-        random_end = pow(10, 64 - len(prefix)) - 1
-        return prefix + str(random.randint(random_begin, random_end))
+    def _new_ui(self, ui: str) -> str:
+        return "2." + str(10**39 + self._randomizer.to_int(ui))

diff --git a/tests/test_anonymize_through_time.py b/tests/test_anonymize_through_time.py
new file mode 100644
index 0000000..1b915a4
--- /dev/null
+++ b/tests/test_anonymize_through_time.py
@@ -0,0 +1,37 @@
+import pydicom
+
+from itertools import filterfalse, tee
+from typing import Callable, Tuple, Union, ValuesView
+
+from dicognito.anonymizer import Anonymizer
+
+from .data_for_tests import load_minimal_instance
+
+
+def test_dataset_anonymizes_same_with_same_seed():
+    anonymizer1 = Anonymizer(seed="SOME_FIXED_SEED")
+    anonymizer2 = Anonymizer(seed="SOME_FIXED_SEED")
+
+    with load_minimal_instance() as dataset1, load_minimal_instance() as dataset2:
+        anonymizer1.anonymize(dataset1)
+        anonymizer2.anonymize(dataset2)
+
+        mismatches, matches = partition(lambda value: value == dataset2[value.tag], dataset1.values())
+
+        assert list(value.name for value in matches)
+        assert not list(value.name for value in mismatches)
+
+
+_DatasetValue = Union[pydicom.DataElement, pydicom.dataelem.RawDataElement]
+
+
+def partition(
+    predicate: Callable[[_DatasetValue], bool],
+    iterable: ValuesView[_DatasetValue],
+    # pytest can't collect the tests when we subscript filterfalse and filter
+) -> Tuple[filterfalse, filter]:  # type: ignore[type-arg]
+
+    "Use a predicate to partition entries into false entries and true entries"
+    # partition(is_odd, range(10)) --> 0 2 4 6 8   and  1 3 5 7 9
+    t1, t2 = tee(iterable)
+    return filterfalse(predicate, t1), filter(predicate, t2)

Support fully-reproducible deidentification
When data on a given cohort is accumulated over long periods, users may wish to run dicognito in multiple passes in order to perform preliminary analyses on the partial dataset. It would be convenient to be able to checkpoint the `Anonymizer` state so that patients seen in previous dicognito runs over the same cohort would have matching anonymized IDs.

Two options occur to me:
1) Use the anonymization map proposed in #124 as a simple checkpoint. I haven't looked at the code yet, so I'm not sure exactly what drawbacks this would have. I think there are some guarantees about the order of dates that might be broken in this case.
2) Serialize everything in the `Anonymizer` and save it to a pickle file. I think this would make starting from a checkpoint 'equivalent' to running in a single pass. It would have the disadvantage of adding another file with sensitive data to manage.
tests/test_anonymize_through_time.py::test_dataset_anonymizes_same_with_same_seed