Over 1 year of ultrasound (US) mass screening, 4,568 newborns were examined consecutively at age 3–1 weeks using both clinical and ultrasonographic examination methods. The US examination involved both the static and dynamic scanning. In total, 25 babies were treated early (5.5 per 1,000), and 6 babies were given late treatment for postnatally developing acetabular dysplasia (1.3 per 1,000). Early treatment was instituted only for hips remarkably distorted anatomically, as could be seen from their shape on the static scan, and for those that were sonographically unstable with an apparent stress displacement. Clinical examination at age 3–4 weeks failed in more than half of all sonographically abnormal cases. In one case, the postnatal development of the hip joint was associated with the worsening of its formation and a developing subluxation despite its primary normal characteristics.