Cyclops syndrome is a loss of knee extension due to impingement of a pedunculated proliferative tissue mass after injury of the anterior cruciate ligament (ACL) with or without reconstruction. We present two patients with a cyclops lesion. One was caused by complete rupture of the posterior cruciate ligament (PCL) without reconstruction. A literature search found no previous report regarding cyclops syndrome after rupture of the PCL without reconstruction. The other was after complete rupture of the ACL without reconstruction. In this article, similar cases caused by complete rupture of the ACL without reconstruction, which were diagnosed and treated by arthroscopic surgery, are also reviewed to discuss their etiology, clinical significance and treatment.