Spontaneous osteonecrosis was diagnosed in ninety knees in which a roentgenographically visible lesion had developed within six months after onset of symptoms. Scintimetry can be used to differentiate between osteonecrosis and other painful conditions of the knee. The prognosis of osteonecrosis is dependent on the size of the radiolucent lesion. In knees with a lesion that is larger than 2.3 square centimeters, osteoarthritis is likely to develop. For forty-two knees the patients were treated with analgesics, partial weight-bearing, and quadriceps exercises. Forty-eight knees were treated surgically. Indications for surgical treatment were persistent severe complaints in spite of conservative treatment and a large osteonecrotic lesion with collapse of the condyle resulting in a varus deformity. The best results were achieved with osteotomy together with arthrotomy, and by knee replacement arthroplasty. Arthrotomy and drilling alone were ineffective.