High resolution ultrasonography of the knee was performed on 82 young patients with clinically suspected Osgood-Schlatter disease and on 30 normal subjects; in 45 pathological cases (55%) comparative X-ray films were taken. The ultrasound pictures were equally or more effective than X-ray images in 45/45 cases; their value was particularly marked for soft tissue study. The typical sonographic changes of the ossification center, of the cartilage, and of the surrounding soft tissues are described and classified, both for Osgood-Schlatter and for Sinding-Larsen-Johansson diseases. These signs are based mainly upon cartilage swelling and edema, fragmentation of the ossification center, thickening of the patellar tendon, and bursitis of the infra-patellar bursa. Ultrasonography is proposed as a simple and reliable method for the diagnosis of knee joint osteochondrosis. The ultrasound picture is also suitable for periodical follow-up the course of the disease.