Examination of the hip joint by computed tomography was evaluated in 34 children admitted to hospital with symptoms of idiopathic increased anteversion of the femoral neck and in 6 children with congenital dislocation of the hip. Anteversion of the femoral neck and the acetabulum was determined from tomographic cross-sections, and femoral anteversion was also determined by conventional radiography. A good correlation was found between these two methods. The relationship between anteversion of the femoral neck and that of the acetabulum was calculated in the 34 patients with idiopathic increased femoral anteversion. In creased anteversion of the femoral neck was not compensated for by a corresponding reduction in the ventral orientation of the acetabulum. In many cases the adjustment of the acetabulum to the femoral head was poor. In some of the patients the degree of external rotation of the hip was greater than would be expected from the relationship between the anteversion of the femoral neck and that of the acetabulum. In these patients the lower limb is forced outwards when walking, causing the femoral head to dislocate laterally and forwards. Based on the results of this study we conclude that calculation of the anteversion, both of the femoral neck and acetabulum, should play an important part in the evaluation of candidates for derotational osteotomy of the femur. Computed tomography yields valuable information in this respect.