The authors review their experience with four patients with congenital deficiency of the anterior cruciate ligament (ACL) who underwent surgical treatment of symptomatic knee instability at a mean age of 15.8 years (range 14-17 years). Associated syndromes included fibular hemimelia, congenital short femur, and an unspecified skeletal dysplasia. All patients had undergone multiple previous realignment and leg lengthening procedures and were skeletally mature at the time of the reconstruction. All four patients underwent ACL reconstruction, and one patient underwent concomitant posterolateral corner reconstruction. One patient required an osteochondral autograft transplant procedure in addition to ACL reconstruction. Hypertrophy of the meniscofemoral ligament of Humphrey was a consistent anatomic finding at surgery. The patients were followed for a mean of 38 months (range 26-58 months) after the reconstruction. The mean preoperative Lysholm II score was 38 (range 28-56); the score had improved to a mean of 81 (range 78-93) at the latest follow-up. The authors conclude that reconstructive surgery is a viable option for restoration of knee stability and function in appropriately selected patients with congenital ACL deficiency.