One hundred patients treated for slipped capital femoral epiphysis (SCFE) were reviewed to evaluate the incidence of bilateral slipping of the epiphysis at an average follow-up time of 32 years. When the patients were examined during adolescence, repeat lateral radiographs of the hips had been obtained by the frog lateral view in 33 patients and by the standardized lateral view according to the method of Billing in 67 patients. At re-examination, 59 patients (59%) were judged to have had a previous bilateral SCFE; in 42 of these 59 patients (71%), slipping of the contralateral hip was asymptomatic. In 23 patients (23%), the diagnosis of bilateral slipping was established at primary admission, in 18 (18%) later during adolescence, and in 18 (18%) not until the patients were reexamined as adults and the primary radiographs were reviewed. The incidence of bilateral slipping was higher in patients in whom the Billing standardized lateral view was used (63%) than in patients in whom the frog lateral view was used (52%). We conclude that the incidence of bilateral slipping of the epiphysis in patients with SCFE is approximately 60% in Sweden. If repeat radiographs of the contralateral hip are obtained with the Billing standardized lateral view until physeal closure occurs, even minor slipping of the epiphysis, which is often advisable to treat, will be apparent.