To evaluate the usefulness of dynamic ultrasonography in documenting treatment efficacy and expecting prognosis in developmental dysplasia of the hip (DDH), 49 DDH hips in 32 patients, who had serial dynamic ultrasonographic examinations by Harcke's method during Pavlik harness treatment, were studied as for the progression of ultrasound findings. Grafs alpha and beta angle were measured in coronal flexion view of dynamic ultrasonography. Among 49 hips, on follow-up radiographs, 44 hips showed normal hip development and 5 hips showed dysplasia. In 44 normal hips, 43 hips had alpha angle of more than 50 degrees and stability on dynamic sonogram at the time of weaning of harness. In contrast, all five hips with residual dysplasia exhibited instability on dynamic stress with comparable alpha angle to that of normal hips. This finding suggests that when the hip matures beyond "delayed ossification"(alpha angle more than 50 degrees) with stability on dynamic sonogram, normal hip development can be expected. But if the hip is continuously unstable, regardless of alpha angle, there can be a possibility of residual dysplasia.