Congenital abnormalities of the femur vary from a deficiency of the entire femur with abnormal development of the pelvis to a hypoplastic femur of normal configuration. Previous classification systems that have focused on either congenital coxa vara, hypoplastic femur, or proximal femoral focal deficiency provide a limited definition of congenital abnormalities and are included within the combined classification system outlined in this text. This system, based on embryological, teratological, biological, and anatomical considerations of 125 patients with 139 affected femora, classifies deficiencies of the proximal end, middle, and distal end of the femur and associated lower extremity abnormalities. Unlike previous classification systems, congenital femoral abnormalities principally involving the middle and distal end of the femur are recognized in distinct classes when appropriate. Associated abnormalities range from severe growth retardation of the tibia and an absent fibula accompanied by a deficiency of ischiopubic structures with an absent acetabulum to a mild tibial and fibular growth retardation. Treatment objectives include pelvic-femoral stability, prosthetic management, extremity length equality, knee stability, ankle and foot stability, and anatomical alignment.