diff --git a/src/textual/css/model.py b/src/textual/css/model.py
index c5efebc153..ed9db4df84 100644
--- a/src/textual/css/model.py
+++ b/src/textual/css/model.py
@@ -108,7 +108,7 @@ def _check_class(self, node: DOMNode) -> bool:
         return True
 
     def _check_id(self, node: DOMNode) -> bool:
-        if not node.id == self._name_lower:
+        if (node._cmp_id is None) or (node._cmp_id.lower() != self._name_lower):
             return False
         if self.pseudo_classes and not node.has_pseudo_class(*self.pseudo_classes):
             return False
diff --git a/src/textual/dom.py b/src/textual/dom.py
index 1a35b07176..54a3d555e9 100644
--- a/src/textual/dom.py
+++ b/src/textual/dom.py
@@ -121,6 +121,7 @@ def __init__(
         self._classes = set()
         self._name = name
         self._id = None
+        self._cmp_id = None
         if id is not None:
             self.id = id
 
@@ -308,6 +309,7 @@ def id(self, new_id: str) -> str:
                 f"Node 'id' attribute may not be changed once set (current id={self._id!r})"
             )
         self._id = new_id
+        self._cmp_id = new_id.lower()
         return new_id
 
     @property
diff --git a/tests/test_query.py b/tests/test_query.py
index 13d5bc5b4e..ebc9c0247c 100644
--- a/tests/test_query.py
+++ b/tests/test_query.py
@@ -18,7 +18,7 @@ class App(Widget):
     app._add_child(help_view)
 
     widget1 = Widget(id="widget1")
-    widget2 = Widget(id="widget2")
+    widget2 = Widget(id="Widget2") # Note case.
     sidebar = Widget(id="sidebar")
     sidebar.add_class("float")
 
@@ -54,7 +54,9 @@ class App(Widget):
         assert list(app.query("#main")) == [main_view]
         assert list(app.query("View#main")) == [main_view]
         assert list(app.query("#widget1")) == [widget1]
+        assert list(app.query("#Widget1")) == [widget1] # Note case.
         assert list(app.query("#widget2")) == [widget2]
+        assert list(app.query("#Widget2")) == [widget2] # Note case.
 
         assert list(app.query("Widget.float")) == [sidebar, tooltip, helpbar]
         assert list(app.query(Widget).filter(".float")) == [sidebar, tooltip, helpbar]