The metaphyseal-diaphyseal angle is the angle created by the intersection of a line through the transverse plane of the proximal tibial metaphysis with a line perpendicular to the long axis of the tibial diaphysis. This angle represents the degree of deformity of the proximal end of the tibia in a patient with clinical bowleg deformity and permits early differentiation between infantile tibia vara and physiological bowleg, before the appearance of the radiographic changes of tibia vara. In twenty-nine of thirty affected extremities with an initial metaphyseal-diaphyseal angle of more than 11.0 degrees, radiographic changes of tibia vara later developed. However, only three of fifty-eight extremities with a metaphyseal-diapyseal angle of 11.0 degrees or less had any of the diagnostic changes. In addition, the ratio of the metaphyseal-diaphyseal angle (proximal tibial metaphyseal deformity) to the tibiofemoral angle (deformity of the entire extremity) showed that approximately 60 per cent of the deformity in tibia vara originates in the proximal metaphysis, whereas only 20 per cent of the deformity in physiological bowing originates there. The metaphyseal-diaphyseal angle allows accurate early diagnosis of bowleg deformity, as well as accurate assessment of its progression.