Instability of the knee is frequently found in association with congenital leg-length discrepancy. We have studied six such patients clinically, radiologically and arthroscopically. Clinical signs of knee instability and significant radiological changes were present in all, and at arthroscopy the anterior cruciate ligament was completely absent in four patients and functionless in the other two. This deficiency appears to be a congenital condition which may predispose to meniscus injury or retropatellar pain; it may also lead to subluxation or dislocation of the knee during leg-lengthening procedures.