The clinical and radiological state of the hips of a group of children with cerebral palsy treated without operation is compared with that in a group treated by operation to correct adduction and flexion deformity and to obtain balanced action in the hip muscles. In the first group, 11 per cent of hips were dislocated, 28 per cent subluxated, 46 per cent dysplastic and 15 per cent normal. In the second group no hip was dislocated, 13 per cent were subluxated, 35 per cent dysplastic and 52 per cent normal. Surgical intervention is indicated clinically for a range of abduction diminishing to less than 45 degrees and–on radiological criteria–for early dysplastic changes, especially a break in Shenton's line, irrespective of the patient's age, severity of involvement or neurological maturity. Prevention of subluxation or dislocation improves function and diminishes the liability to develop a painful hip in adolescence or early adult life.