A mediopatellar plica (synovial fold) of the knee-joint may develop without recognisable cause in adolescents or young adults, predominantly females. It leads to pain on pressure over the medial knee compartment, sudden or "springing" intraarticular movements and pseudolocking of the joint. Similar plicae occur after traumatic joint contusion, with meniscus disease, or more rarely with arthrosis deformans. Histologically they are characterized by band-like fibrosed evaginations of the synovial membrane and of the synovial fat and connective tissue into the joint spaces. The following therapeutic suggestions, based on the personal experience of 15 cases, are made in the knowledge that significant inflammatory or proliferative arthritic changes can be excluded: the plica can be cut through under arthroscopy; chondromalacial defects, directly or indirectly caused by plical rubbing, of the medial femoral condyle and the medial patella can be removed, also under arthroscopic control, with an electric razor. Arthrotomy is no longer needed in most cases.