1. Fifteen congenitally hyperextended and anteriorly subluxated knees of eleven patients have been described. All deformities were resistant to conservative treatment and required open reduction.

2. Every patient had associated congenital abnormalities of the hip, seven patients had club-foot deformities, one had calcaneovalgus deformities of the feet at birth, and seven were considered to have arthrogryposis multiplex congenita.

3. Fibrous replacement of the quadriceps was encountered in all of the knees operated on. The lateral portion of the quadriceps was characteristically affected and the medial portion was relatively spared.

4. Lateral placement of the patella and its extensor mechanism was found in over half of the cases.

5. Quadriceps lengthening, anterior capsulotomy, and mobilization of the collateral ligaments were required to effect reduction. The amount of available normal-appearing muscle varied; that which was present was aligned to exert a direct axial pull on the patella in the line of the femur.

6. Twelve knees were followed for from two to twenty-five years (mean, twelve years) after surgery.

7. Seven knees were considered to have good or excellent functional results, three were graded fair, and two poor (one requiring arthrodesis). No patient had any residual hyperextension deformity, and more active extension was obtained than was anticipated from the limited amount of normal muscle observed at operation.

8. The lasting results found in this study suggest that if a congenitally hyperextended knee with subluxation fails to respond to conservative treatment, early open reduction is indicated.