A retrospective study of the development of the hip opposite a congenitally dislocated hip was carried out to identify at an early age those hips which would develop abnormally. Recognised radiological measurements were used showing the development of the joints with age. The development was compared with that of a control group of normal hips. Single measurements were shown to be unreliable in predicting the development of the hip. A simple hip ratio is proposed which predicts, at an early age and with a high degree of accuracy, the developmental outcome.