We examined twenty-five patients who had a unilateral or bilateral slip of the capital femoral epiphysis and determined the degree of anteversion of the thirty-nine involved hips with computerized axial tomography. Thirteen patients (eighteen hips) were seen at the time of the original diagnosis (Group I), and twelve patients (twenty-one hips) were seen one to seven years after operative treatment (Group II). The mean amount of anteversion for all of the involved hips was +1.0 +/- 8.2 degrees. The mean amount of anteversion for the Group-I hips was -0.7 +/- 7.4 degrees and the mean amount for the Group-II hips was 2.5 +/- 8.7 degrees. The mean amount of anteversion for the hips in both Groups I and II was less than the predicted mean amount for individuals of the same age. The mean amount of anteversion of the unaffected hips of our patients who had a unilateral slip was +6.3 +/- 8.2 degrees. The amount of internal rotation of the hip in extension exceeded the amount when it was in flexion in all of the patients. A decreased angle of femoral anteversion appears to be specifically associated with the development of slipped capital femoral epiphysis. The mechanical forces that act across the proximal femoral physis may be altered by this rotational abnormality, and this may lead to an increased shear stress that ultimately causes failure of the growth plate.