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A study of acetabular development in 148 patients with 185 congenitally dislocated hips treated by closed reduction from 1940 to 1968 showed that eight years after reduction the acetabular index was normal (less than 20 degrees) in 49.2 per cent, satisfactory (from 20 to 24 degrees) in 34 per cent, and poor (more than 24 degrees) in 16.7 per cent. Early treatment led to the best acetabular development. Significant and continuing improvement of the acetabular index was observed between two and eight years after reduction when the femoral head remained concentrically reduced. Failure to maintain proper reduction and necrosis of the femoral head led to poor results.