We studied the cases of twenty-one patients with the syndrome of thrombocytopenia and absent radius. Patients with this syndrome usually have associated intra-articular dysplasia of the knee joint bilaterally, causing genu varum and a flexion and torsional deformity that become manifest in the first or second year of life. Usually there is progression of the deformities during growth and therefore treatment with braces or operations, or both, will frequently be needed. Despite correctional osteotomy, the varus angulation and internal tibial rotation have a natural tendency to recur, possibly because of the intra-articular deformity that involves the femoral and tibial condyles. After skeletal maturity has been reached, minimum progression of the deformity has been observed.