diff --git a/pytype/tests/test_test_code.py b/pytype/tests/test_test_code.py
index 45cbd374a..894816712 100644
--- a/pytype/tests/test_test_code.py
+++ b/pytype/tests/test_test_code.py
@@ -62,8 +62,23 @@ def test_foo(self):
           assert_type(x, str)
     """)
 
+  def test_narrowed_type_from_assert_isinstance(self):
+    # assertIsInstance should narrow the original var's bindings if possible.
+    self.Check("""
+      import unittest
+      from typing import Union
+      class A:
+        pass
+      class B(A):
+        pass
+      class FooTest(unittest.TestCase):
+        def test_foo(self, x: Union[A, B, int]):
+          self.assertIsInstance(x, A)
+          assert_type(x, Union[A, B])
+    """)
+
   def test_new_type_from_assert_isinstance(self):
-    # assertIsInstance should create a var with a new type even if it is not in
+    # assertIsInstance should create a var with a new type if it is not in
     # the original var's bindings.
     self.Check("""
       import unittest
diff --git a/pytype/vm.py b/pytype/vm.py
index 4de0cf8f6..cc5f21fd7 100644
--- a/pytype/vm.py
+++ b/pytype/vm.py
@@ -3021,12 +3021,27 @@ def _narrow(self, state, var, pred):
     return self._store_new_var_in_local(state, var, out)
 
   def _set_type_from_assert_isinstance(self, state, var, class_spec):
+    """Set type of var from an assertIsInstance(var, class_spec) call."""
     # TODO(mdemello): If we want to cast var to typ via an assertion, should
-    # we check that at least one binding of var is compatible with typ?
+    # we require that at least one binding of var is compatible with typ?
     classes = []
     abstract_utils.flatten(class_spec, classes)
+    ret = []
+    # First try to narrow `var` based on `classes`.
+    for c in classes:
+      m = self.ctx.matcher(state.node).compute_one_match(
+          var, c, keep_all_views=True, match_all_views=False)
+      if m.success:
+        for matched in m.good_matches:
+          d = matched.view[var]
+          if isinstance(d.data, abstract.Instance):
+            ret.append(d.data.cls)
+
+    # If we don't have bindings from `classes` in `var`, instantiate the
+    # original class spec.
+    ret = ret or classes
     instance = self.init_class(
-        state.node, self.ctx.convert.merge_values(classes))
+        state.node, self.ctx.convert.merge_values(ret))
     return self._store_new_var_in_local(state, var, instance)
 
   def _check_test_assert(self, state, func, args):