Two hundred and seventy hips of 220 patients were treated with the Pavlik harness for congenital dislocation. Two hundred and thirty-three hips were dislocated, and thirty-seven hips had acetabular dysplasia. Of the 233 dislocated hips, thirteen (6 per cent) were not reduced. In one hundred and eighty-four hips that were successfully reduced, avascular necrosis did not develop. Avascular necrosis was observed in thirty-six (16 per cent) of the 220 reduced hips. Of these, twenty-seven hips were followed for five to twenty years (average, nine years). The result was excellent or good in eighteen hips (67 per cent), fair in three (11 per cent), and a failure in six (22 per cent). Avascular necrosis was seen in one of the thirty-seven hips that had acetabular dysplasia, and it was found even in the normal hip of a patient who had unilateral dislocation. In this study, the amount of displacement was represented by distances a and b, as defined by Yamamuro and Chene. Distance a is the length from the middle point of the proximal metaphyseal border of the femur to the Y-line, and distance b is the distance from the same point to the lateral margin of the ischium. When distance a was more than four millimeters, the over-all rate of reduction was 97 per cent (209 of 215 hips), ranging from 96 per cent (eighty-two of eighty-five hips) when distance a was eight to ten millimeters to 100 per cent (twenty-nine hips) when distance a was more than ten millimeters.(ABSTRACT TRUNCATED AT 250 WORDS)