diff --git a/CHANGELOG.md b/CHANGELOG.md
index e6d3336..caf1254 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - Request info in response.
 
+### Changed
+- **BREAKING**: Snake case `runtimeInMilliseconds`.
+
 ## [0.5.1] - 2023-03-10
 ### Added
 - Uniqueness to custom "about" information.
diff --git a/README.md b/README.md
index 4a6366f..2839819 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@ The endpoint will return data in JSON.
             "slug": "database",
             "group": "core",
             "description": "Check all database connections.",
-            "runtimeInMilliseconds": 10,
+            "runtime_in_milliseconds": 10,
             "result": {
                 "value": 1,
                 "message": "Connected to all databases.",
diff --git a/src/Repository.php b/src/Repository.php
index 0957700..54c0829 100644
--- a/src/Repository.php
+++ b/src/Repository.php
@@ -90,7 +90,7 @@ private function checkToArray(Check $check): array
             'group' => $check->group ?? 'other',
             'description' => $check->description,
             'result' => $check->run()->toArray(),
-            'runtimeInMilliseconds' => $start->diffInMilliseconds(),
+            'runtime_in_milliseconds' => $start->diffInMilliseconds(),
         ];
     }
 
diff --git a/tests/RepositoryTest.php b/tests/RepositoryTest.php
index 0b1ac17..7a2edc1 100644
--- a/tests/RepositoryTest.php
+++ b/tests/RepositoryTest.php
@@ -51,7 +51,7 @@ public function test_invoke_returns_correct_default_information()
                     'slug' => 'test-check',
                     'group' => 'other',
                     'description' => 'A test check',
-                    'runtimeInMilliseconds' => 100,
+                    'runtime_in_milliseconds' => 100,
                     'result' => [
                         'value' => null,
                         'message' => 'Looking good.',