Real-time ultrasound was assessed as a means of evaluating osteoarthritis of the knee. The knee was maintained in complete flexion so as to expose the weight-bearing portion of the femoral condylar cartilage. After establishing the validity of the method in an excised bovine knee, 7 asymptomatic individuals, 10 arthritic patients, and several patients with other conditions were examined. The results indicated that sonography can be used to measure the thickness of the articular cartilage in man, as well as to detect changes in its surface and internal characteristics. Early changes of arthritis may be revealed in this manner.