Deformities of the hip in adults who have sickle-cell disease and had avascular necrosis in childhood. A natural history of fifty-two patients. We report the natural course of the hip in fifty-two patients (ninety-five hips) who had sickle-cell disease and had had avascular necrosis in childhood. There were twenty-one African, twenty-one West Indian, and ten Mediterranean patients. At the most recent follow-up examination (at an average duration of nineteen years after the onset of the disease), 80 per cent of the hips that had been affected by avascular necrosis during childhood were painful and had permanent damage with regard to decreased mobility, limb-length discrepancy, and an abnormal gait. When the patients were evaluated, at an average age of thirty-one years, fifteen hips (16 per cent) had had an operation for progressive disability and sixty (63 per cent) had major problems because of pain. Of the twenty hips (21 per cent) that were not painful, five were in patients who had an abnormal gait, with decreased agility. The mean Iowa hip-rating score at the most recent follow-up examination was 73 points (range, 30 to 100 points). Correlations were found between the hip score and the patient's age at the onset of the disease and at the latest follow-up, between the hip score and degenerative changes in the hip, and between degenerative changes and radiographic evidence of deformity of the hip. 