Five knees with congenital dislocation were explored. The cruciate ligaments were found to be absent or hypoplastic. The results obtained by reconstructing an anterior cruciate ligament appear to be good. On the basis of dissections of foetal knees we believe that the cruciate ligaments are the main structures preventing an anterior dislocation of the knee in early foetal life; this deviation from the adult pattern is due to the bony configuration of the foetal knee. We therefore postulate that the basic defect in congenital dislocation of the knee is an absence or hypoplasia of the cruciate ligaments.