The long-term results (9-87 months) of the arthroscopic treatment of unilateral mediopatellar synovial plica of the knee in 102 athletes are reported. In all patients a large and/or fibrotic inflamed mediopatellar plica impinging on the femoral condyle on knee flexion was identified and trimmed. Chondromalacia patellae was detected in 38 patients. The impingement lesion was seen in 13 knees. In one patient, a minor tear of the lateral meniscus was trimmed, and one patient was found to suffer from a clinically asymptomatic partial tear of the anterior cruciate ligament. Isometric strength testing of the quadriceps revealed marked decrease at the second postoperative week, with subsequent gradual recovery. The results obtained justify arthroscopic treatment in a young, extremely active population.