Redislocation of the femoral head may be occured after its operative reduction in the congenital dislocation of the hip, therefore, it is greatly important to disclose the causes of the redislocation in order to avoid this every complication and improve the curative effect of this operation. Seven cases of redislocation from 106 cases (128 sides) of the congenital dislocation of the hip which had been reduced operatively were studied with relative measurements of the hip joints on roentgenogram, associated their pathologic conditions described in operation. The results showed that, in these cases, there were (0.843 +/- 0.692) cm upward displacement of the femoral head beyond the horizontal Y line, (68.86 +/- 0.692) degree of the femoral anteversion, the more lateral displacement of the femoral head compared to the opposite side and the acetabular index increasing up to (33.86 +/- 3.72) degree from (26.14 +/- 2.73) degree of the operative correction. These phenomena indicate that the redislocation after operative reduction of the femoral head in congenital dislocation of the hip is mainly related to four causes which include the existence of large pressure between acetabulum and femoral head, the uncorrected abnormal femoral anteversion, the residue of the pathologic tissues in the acetabulum and the reascending of the acetabular index having been corrected in operation.