Results of animal experiments and then of clinical study of newborn infants suggested that the main mechanical factor causing hip dislocation is prolonged tension on the hamstrings and iliopsoas and that each of these muscles has a synergistic effect in the production of hip dislocation during the perinatal period. It was, therefore, presumed that the high incidence of congenital dislocation of the hip (CDH) in Japan might be due mainly to the Japanese tradition of forcibly maintaining the legs of infants in an extended position with a "swathing diaper." The incidence of CDH in Japanese infants prior to 1965 was as high as 1.1%-3.5%. In an attempt to reduce the incidence of CDH, examination of newborns was performed on a national scale. However, early screening methods for hip abnormalities were not adequately reliable in Japan to allow identification of conditions that would later become CDH. In 1975 a national campaign to avoid prolonged extension of the hips and knees of infants during the early postnatal period was initiated. As a result, there has been a remarkable reduction in the incidence of CDH in infants, to less than 0.2%.