We used ultrasound to investigate the anatomy and stability of the hip during the phase of joint stabilisation after closed reduction of unilateral late CDH in 15 infants aged from two to 15 months. Conservative treatment by splintage in abduction led to complete stability in all hips in from three to 13 weeks. Immediately after reduction, interposed soft tissue caused lateralisation of the femoral head, but this progressively disappeared, and the initially smaller femoral head on the dislocated side regained normal size. Ultrasonic evaluation of the hip in late CDH is a valuable and safe tool; it reduces the need for radiographic examination and improves treatment by visualising the actual pathology. The more frequent use of ultrasound can reduce the need for open reduction in the age group that we studied.