We report a case of an apparent avulsion fracture of the posteromedial margin of the medial plateau of the tibia. This was associated with a tear of the medial meniscus and rupture of the anterior cruciate ligament (ACL). This triad has been previously reported, and the plateau fracture was related to the insertion of the semimembranosus tendon. The detailed investigation of our case and some experiments on cadaver knees showed that the injury was not an avulsion fracture but was produced by varus and external rotation forces on a flexed knee. It was due to the anterior subluxation of the medial tibial plateau after rupture of the ACL.