In order to predict the probability of collapse of a femoral head in which there is avascular necrosis, we used magnetic resonance imaging to evaluate sixty-six hips (fifty patients) in which avascular necrosis was in the early stages and in which collapse of the head was not yet visible. The hips were followed radiographically for an average of forty-nine months (range, sixteen to eighty-four months), and with magnetic resonance imaging for an average of forty-four months (range, twelve to seventy-three months). Twenty-one (32 per cent) of the femoral heads had collapsed by thirty-two months. Of the twenty-three femoral heads in which necrosis involved at least one-fourth of the diameter of the head and encompassed at least two-thirds of the major weight-bearing area, seventeen (74 per cent) had collapsed by thirty-two months.