The data on 155 hips in 124 patients who had slipped capital femoral epiphysis were retrospectively reviewed at a mean follow-up of forty-one years after the onset of symptoms. The slips were classified, by the duration of symptoms, as acute, chronic, or acute on chronic. As determined by the head-shaft angle, 42 per cent of the slips were mild; 32 per cent, moderate; and 26 per cent, severe. Reduction was performed in thirty-nine hips, and realignment was done in sixty-five hips. Treatment of chronic slips included symptomatic only in 25 per cent of the hips, a spica cast in 30 per cent, pinning in 24 per cent, and osteotomy in 20 per cent. The Iowa hip-rating and the radiographic classification of degenerative joint disease were determined at follow-up; both worsened with increasing severity of the slip and when reduction or realignment had been done. Osteonecrosis (12 per cent) and chondrolysis (16 per cent) also were more common with increasing severity of the slip and when reduction or realignment had been performed; both led to a poor result. Deterioration over time was most marked with increasing severity of the slip. The natural history of the malunited slip is mild deterioration related to the severity of the slip and complications. Techniques of realignment are associated with a risk of appreciable complications and adversely affect the natural history of the disease. Regardless of the severity of the slip, pinning in situ provided the best long-term function and delay of degenerative arthritis, with a low risk of complications.