A congenital, unilateral, fixed flexion deformity in a neonate was diagnosed as a congenital absence of the knee. A single cartilage mass, with fusion of the lower femoral and upper tibial ossification centres, was demonstrated by imaging studies. This condition has been reported in the literature only once before. Surgery on our patient, which was performed at the age of two years, consisted of separation of the fused cartilaginous anlage and gradual correction of the deformity using an Ilizarov frame.