Central to the current treatment of dysplasia of the hip (DDH) is the concept of congruent reduction of the hip. If the femoral head is aspherical in DDH, this concept needs reconsideration. MRI scans are used to examine the femoral head in children. Diameters of 14 DDH and 12 normal hips were measured in three planes by eight observers on two occasions. Femoral head sphericity was determined using the mathematical concept of eccentricity. In DDH, the femoral head was less spherical, most marked in the coronal plane, yielding a ‘rugby ball’-shaped femoral head. Accordingly, concentric reduction of the femoral head in DDH is impossible.