We evaluated retrospectively the 5-year outcome of 13 patients with an initially undiagnosed and thus untreated anterior cruciate ligament rupture. At follow-up, these patients experienced decreased knee function and moderate to severe knee instability. They had attempted to reduce their discomfort by decreasing their activity level during the follow-up period. Immediately after the 5-year examination, 6 of the patients decided to have a late reconstruction of the ruptured anterior cruciate ligament. These results agree with previous reports of the natural course of anterior cruciate ligament ruptures.