I have prospectively studied 71 consecutive patients (75 hips) with late-diagnosed DDH (developmental dysplasia of the hip) treated uniformly with special reference to the development of the femoral head and the acetabulum. The age at the start of treatment was 10 (2-64) months. The follow-up time was 11 (6-18) years. After tenotomy and traction, closed reduction failed in 7 hips. These cases were treated by open reduction with or without Salter innominate osteotomy-in 2 hips femoral osteotomy was also done (shortening, varus and derotation). 1 hip subluxated and 1 re-dislocated after closed reduction. Avascular necrosis occurred in 4 hips and additional surgery was required in 12 hips--11 Salter osteotomies, 1 varus femoral osteotomy. In the first year after reduction, the acetabular angle improved rapidly--faster in the younger children. When treatment started between 12 and 22 months, the improvement was slower and the final outcome more unpredictable. The femoral head continued to grow irrespective of the age at reduction and became normal in almost all cases. Salter's innominate osteotomy stabilized hips after open reduction and gave excellent results in cases with an increasing acetabular angle. At the last re-examination, all but 2 patients were asymptomatic. Radiographically, 65 hips were rated Severin group I, 9 group II and 1 group III.