The thickness of the acetabular floor and the horizontal diameter of the femoral head often appear enlarged on X-ray films of congenitally dislocated hips, particularly in the later stages. These distances were, therefore, measured and compared with the normal side in 49 strictly unilateral cases. Increased floor thickness was an early change and usually persisted, often in spite of a satisfactory reduction. Increased head diameter was detectable later and these two features together increased the distance between the centre of the femoral head and the midline of the body. This lateral displacement, even when the head is well covered, increases abductor effort and force transmitted to the femoral head on weight bearing and may have an adverse effect on the joint's long term future.