The cases of nine patients (eleven hips) with idiopathic chondrolysis of the hip were studied. Seven of the patients were white and two were Hispanic. The age at onset ranged from eight to sixteen years (mean, 11.5 years). Four patients were boys and five were girls. All patients had a decreased passive range of motion of the hip, and radiographic examination showed regional osteoporosis, premature closure of the femoral capital physis, narrowing of the joint space, and lateral overgrowth of the femoral head on the neck. All laboratory examinations were negative for evidence of infection or rheumatoid arthritis. An arthrotomy was done in seven patients. Specimens of the synovial tissue showed no growth on culture, and the histological studies revealed only minimum signs of inflammation. Histological studies of the articular cartilage were normal. Treatment consisted of administration of acetylsalicylic acid in therapeutic dosages to maintain a blood salicylate level of fifteen to twenty-five milligrams per cent, active non-loading exercise of the hip, protected weight-bearing with crutches, short-term traction to overcome or relieve contractures after biopsy, iliopsoas tenotomy or lengthening in three patients, and an adductor myotomy in one patient. At follow-up, 2.3 to 9.4 years after onset (mean, 6.2 years), six patients had either no symptoms or only minor intermittent discomfort in the hip. On radiographic examination, although these six patients had restoration of the joint space they did have lateral overgrowth of the femoral head (lateral buttressing) and overgrowth of the lateral acetabular margin (lateral osteophyte). In three patients who had disabling pain, joint deterioration was evident on the radiographic examination. One of these patients had a resurfacing arthroplasty to relieve pain.