diff --git a/CHANGELOG b/CHANGELOG
index 8787808..147b302 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -6,6 +6,8 @@ Changelog
 
 Bugfixes and documentation improvements.
 
+- Bug #26 - Prevented computation of virtual file's size, unless the file
+  wrapper implements was_modified_since() method.
 - Bug #34 - Improved support of files that do not implement modification time.
 - Bug #35 - Fixed README conversion from reStructuredText to HTML (PyPI).
 
diff --git a/django_downloadview/views.py b/django_downloadview/views.py
index ab1a0e8..8eee5d9 100644
--- a/django_downloadview/views.py
+++ b/django_downloadview/views.py
@@ -173,6 +173,27 @@ class VirtualDownloadView(BaseDownloadView):
         """Return wrapper."""
         raise NotImplementedError()
 
+    def was_modified_since(self, file_instance, since):
+        """Delegate to file wrapper's was_modified_since, or return True.
+
+        This is the implementation of an edge case: when files are generated
+        on the fly, we cannot guess whether they have been modified or not.
+        If the file wrapper implements ``was_modified_since()`` method, then we
+        trust it. Otherwise it is safer to suppose that the file has been
+        modified.
+
+        This behaviour prevents file size to be computed on the Django side.
+        Because computing file size means iterating over all the file contents,
+        and we want to avoid that whenever possible. As an example, it could
+        reduce all the benefits of working with dynamic file generators...
+        which is a major feature of virtual files.
+
+        """
+        try:
+            return file_instance.was_modified_since(since)
+        except (AttributeError, NotImplementedError):
+            return True
+
 
 class HTTPDownloadView(BaseDownloadView):
     """Proxy files that live on remote servers."""
diff --git a/tests/views.py b/tests/views.py
index 77ebca4..a98e2a9 100644
--- a/tests/views.py
+++ b/tests/views.py
@@ -48,3 +48,36 @@ class DownloadMixinTestCase(unittest.TestCase):
                 mixin = views.DownloadMixin()
                 since = mock.sentinel.since
                 self.assertTrue(mixin.was_modified_since(file_wrapper, since))
+
+
+class VirtualDownloadViewTestCase(unittest.TestCase):
+    """Test suite around
+    :py:class:`django_downloadview.views.VirtualDownloadView`."""
+    def test_was_modified_since_specific(self):
+        """VirtualDownloadView.was_modified_since() delegates to file wrapper.
+
+        """
+        file_wrapper = mock.Mock()
+        file_wrapper.was_modified_since = mock.Mock(
+            return_value=mock.sentinel.from_file_wrapper)
+        view = views.VirtualDownloadView()
+        since = mock.sentinel.since
+        return_value = view.was_modified_since(file_wrapper, since)
+        self.assertTrue(return_value is mock.sentinel.from_file_wrapper)
+        file_wrapper.was_modified_since.assert_called_once_with(since)
+
+    def test_was_modified_since_not_implemented(self):
+        """VirtualDownloadView.was_modified_since() returns True if file
+        wrapper does not implement ``was_modified_since()``."""
+        file_wrapper = mock.Mock()
+        file_wrapper.was_modified_since = mock.Mock(side_effect=AttributeError)
+        modified_time = mock.PropertyMock()
+        setattr(file_wrapper, 'modified_time', modified_time)
+        size = mock.PropertyMock()
+        setattr(file_wrapper, 'size', size)
+        view = views.VirtualDownloadView()
+        since = mock.sentinel.since
+        result = view.was_modified_since(file_wrapper, since)
+        self.assertTrue(result is True)
+        self.assertFalse(modified_time.called)
+        self.assertFalse(size.called)