Upper femoral osteotomy is a recognised treatment for selected patients with Perthes' disease. The results of this procedure were investigated at skeletal maturity in 44 patients (48 hips). The indication for operation was Catterall group II, III, and IV hips with 'head-at-risk' signs. Harris and Iowa scores were calculated clinically, and each hip was assigned radiographically to one of the five Stulberg classes, its initial Catterall grading checked and other relevant indices measured. Results showed excellent clinical function. Shortening was present in 14 hips (29%) and a positive Trendelenburg's sign was seen in 12 (25%). On radiographic assessment 58% of hips were Stulberg class I or II, with a good prognosis. The results of femoral osteotomy were better than those for conservatively treated hips in all age groups except those under five years.