diff --git a/CHANGELOG.md b/CHANGELOG.md
index 577480f8..45a07ab3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@
 ### Bug fixes
 
 - **The Elasticsearch/OpenSearch aggregate dashboards' over-time charts (and the Grafana ES dashboard's summary pies and time series) bucketed on the multi-valued `date_range` field**; a date histogram counts a report once per value, double-counting any report whose begin and end dates fall in different buckets. All date histograms and time-range filters now use the single-valued `date_begin`, matching the report-begin semantics of the PostgreSQL (`begin_date`) and Splunk (`_time` = interval begin) dashboards.
+- **Aggregate-report policy and authentication result words are now normalized to lowercase** ([#288](https://github.com/domainaware/parsedmarc/issues/288)): reporters that emit mixed-case values such as `Pass` no longer create duplicate result categories in outputs and dashboards.
 
 ## 10.2.4
 
diff --git a/parsedmarc/__init__.py b/parsedmarc/__init__.py
index 96e6dd90..b02ea901 100644
--- a/parsedmarc/__init__.py
+++ b/parsedmarc/__init__.py
@@ -181,6 +181,15 @@ def _text(value: Any) -> str | None:
     return value
 
 
+def _normalize_result_word(value: Any) -> Any:
+    """Lowercase a reporter-supplied enum word; RFC 7489 Appendix C and
+    RFC 9990 define result/disposition types as lowercase tokens. Non-string
+    values (e.g. xmltodict dicts from attribute-bearing elements) pass
+    through unchanged.
+    """
+    return value.lower() if isinstance(value, str) else value
+
+
 def _bucket_interval_by_day(
     begin: datetime,
     end: datetime,
@@ -422,11 +431,12 @@ def _parse_report_record(
         "policy_override_reasons": [],
     }
     if "disposition" in policy_evaluated:
-        new_policy_evaluated["disposition"] = policy_evaluated["disposition"]
-    if "dkim" in policy_evaluated:
-        new_policy_evaluated["dkim"] = policy_evaluated["dkim"]
-    if "spf" in policy_evaluated:
-        new_policy_evaluated["spf"] = policy_evaluated["spf"]
+        new_policy_evaluated["disposition"] = _normalize_result_word(
+            policy_evaluated["disposition"]
+        )
+    for key in ("dkim", "spf"):
+        if key in policy_evaluated:
+            new_policy_evaluated[key] = _normalize_result_word(policy_evaluated[key])
     reasons = []
     spf_aligned = (
         policy_evaluated["spf"] is not None
@@ -505,7 +515,7 @@ def _parse_report_record(
                     )
                 new_result["selector"] = "none"
             if "result" in result and result["result"] is not None:
-                new_result["result"] = result["result"]
+                new_result["result"] = _normalize_result_word(result["result"])
             else:
                 new_result["result"] = "none"
             new_result["human_result"] = _text(result.get("human_result"))
@@ -521,7 +531,7 @@ def _parse_report_record(
             else:
                 new_result["scope"] = "mfrom"
             if "result" in result and result["result"] is not None:
-                new_result["result"] = result["result"]
+                new_result["result"] = _normalize_result_word(result["result"])
             else:
                 new_result["result"] = "none"
             new_result["human_result"] = _text(result.get("human_result"))
diff --git a/samples/aggregate_invalid/report_with_upper_cased_pass.xml b/samples/aggregate_invalid/report_with_upper_cased_pass.xml
index 406ab6c9..14cc7592 100644
--- a/samples/aggregate_invalid/report_with_upper_cased_pass.xml
+++ b/samples/aggregate_invalid/report_with_upper_cased_pass.xml
@@ -21,7 +21,7 @@
    <source_ip>23.104.41.189</source_ip>
    <count>1</count>
    <policy_evaluated>
-    <disposition>none</disposition>
+    <disposition>None</disposition>
     <dkim>Pass</dkim>
     <spf>Pass</spf>
    </policy_evaluated>
diff --git a/tests/test_init.py b/tests/test_init.py
index 4553061f..7eb1cfd5 100644
--- a/tests/test_init.py
+++ b/tests/test_init.py
@@ -166,6 +166,25 @@ class Test(unittest.TestCase):
                 )
             print("Passed!")
 
+    def testAggregateResultWordsAreLowercase(self):
+        """Reporter-supplied result words are lowercased at ingest; RFC 7489
+        Appendix C and RFC 9990 define the result and disposition types as
+        lowercase enum tokens (issue #288).
+        """
+        result = parsedmarc.parse_report_file(
+            "samples/aggregate_invalid/report_with_upper_cased_pass.xml",
+            offline=True,
+        )
+        assert result["report_type"] == "aggregate"
+        report = cast(AggregateReport, result["report"])
+        record = report["records"][0]
+
+        self.assertEqual(record["policy_evaluated"]["dkim"], "pass")
+        self.assertEqual(record["policy_evaluated"]["spf"], "pass")
+        self.assertEqual(record["auth_results"]["dkim"][0]["result"], "pass")
+        self.assertEqual(record["auth_results"]["spf"][0]["result"], "pass")
+        self.assertEqual(record["policy_evaluated"]["disposition"], "none")
+
     def testEmptySample(self):
         """Test empty/unparasable report"""
         with self.assertRaises(parsedmarc.ParserError):