Twenty-five radiographically proven cases of osteochondritis dissecans of the femoral condyles were examined by real-time sonography using a 5 MHz linear transducer. The medial condyle was involved in 19 cases and the lateral in 6. The size of the lesions varied from a few mm to more than 30 mm in diameter. The fragmentation of the subchondral bone was demonstrated in all cases, and furthermore the osteochondral fragment and the condition of the overlying articular cartilage could be assessed. The exact indications for the technique have not yet been defined, but awaits further clinical studies.