We studied the natural history of nontraumatic avascular necrosis of the femoral head (ANFH) in 115 hips in 87 patients, 69 steroid-induced, 21 related to misuse of alcohol and 25 idiopathic. The average length of follow-up was over five years. Collapse occurred most often when the focus of bone necrosis occupied the weight-bearing surface of the femoral head. Flatness of the head due to subchondral fracture was an early manifestation of collapse. Classification into six types based upon the radiographic findings provided an accurate prognosis for individual cases of ANFH which is useful in planning treatment and in assessing its outcome.