diff --git a/atlas/templates/atlas/collection_detail.html b/atlas/templates/atlas/collection_detail.html
index 6d97ad12..5ca9e1bb 100644
--- a/atlas/templates/atlas/collection_detail.html
+++ b/atlas/templates/atlas/collection_detail.html
@@ -20,9 +20,9 @@
 
                         {% include 'exam_notes.html' %}
                         {% if casesdetails|length > 20 %}
-                            <div class="alert alert-warning alert-dismissible fade show" role="alert">
+                            <div id="collection-warning-{{ collection.pk }}" class="alert alert-warning alert-dismissible fade show" role="alert">
                                 <strong>Performance warning:</strong> This collection contains {{ casesdetails|length }} cases — in some situations this may adversely affect performance, all features will still work but page loading may be slower. Consider splitting into smaller collections if performance is an issue.
-                                <button type="button" class="btn-close" data-bs-dismiss="alert" aria-label="Close"></button>
+                                <button id="collection-warning-close-{{ collection.pk }}" type="button" class="btn-close" data-bs-dismiss="alert" aria-label="Close"></button>
                             </div>
                         {% endif %}
 
@@ -164,4 +164,25 @@
         </div>
     </div>
 
+
+    <script>
+        document.addEventListener('DOMContentLoaded', function() {
+            try {
+                var pk = '{{ collection.pk }}';
+                var key = 'collection_warning_dismissed_' + pk;
+                var alertEl = document.getElementById('collection-warning-' + pk);
+                if (!alertEl) return;
+                if (localStorage.getItem(key) === '1') {
+                    alertEl.remove();
+                    return;
+                }
+                var btn = document.getElementById('collection-warning-close-' + pk);
+                if (btn) {
+                    btn.addEventListener('click', function() {
+                        try { localStorage.setItem(key, '1'); } catch (e) { /* ignore */ }
+                    });
+                }
+            } catch (e) { console.warn('collection warning persistence error', e); }
+        });
+    </script>
 {% endblock %}