We report a case of valgus knee deformity with patellar dislocation caused by infantile septic arthritis. The patient, a boy, had a congenital heart malformation. He had developed infantile septic arthritis of the knee joint at 1 month of age, and the distal lateral epiphysis of the femur had been affected. At 6 years of age, valgus deformity appeared, and therefore we performed deformity correction and leg lengthening with an Ilizarov fixator. However, the deformity recurred and the patella became dislocated laterally 3 years after initial surgery. In a second operation, we performed osteotomy at two sites of the femur. Distally, we corrected the valgus deformity and reduced the patella by changing the rotation of the femur and overlapping the medial soft tissue. Proximally, we corrected the retroversion and performed lengthening on a gradual basis. In the left leg, the preoperative mLDFA was 110° and the postoperative mLDFA was improved to 90°. As the deformity will likely relapse because of the patientｾs age, we plan to achieve realignment again by corrective surgery.