Clinical and roentgenographic examination of the hip was performed in 114 patients with Down's syndrome to study range of motion, roentgenographic anatomy, and incidence of hip pathology. The study found increased external rotation of the hip. Roentgenographic studies demonstrate that, in comparison with a normal acetabulum, the acetabulum of a patient with Down's syndrome is deep, more horizontally placed, and has increased anteversion. The proximal femur of a patient with Down's syndrome has a normal neck-shaft angle and a moderate increase in anteversion. Of the patients, 7.9% had some hip abnormality, including dysplasia, dislocation, avascular necrosis, or slipped capital femoral epiphysis.