To elucidate the cause of tibia vara, finite element analysis of the proximal tibia was used to investigate the stresses occurring in the physeal plate during one-legged stance in 2- and 5-year-old children. A modification of the method of Kettlekamp and Chao was used to assign forces to the medial and lateral plateaus and lateral ligament. Stresses were calculated in the physeal plate for the two age groups as a function of degree of varus and body weight. Our results show that increasing varus resulted in increasing compressive stress in the medial tibial physis to a level seven times normal at 30 degrees of varus. Further, tensile stresses determined in the lateral tibial physis were increased above normal. Changes were more marked in the obese child and in the 5 year old. Using the data from Strobino et al. it appears that in the 2 year old 20 degrees of varus resulted in forces sufficient to retard growth. In the 5 year old, however, 10 degrees of varus resulted in borderline forces in a child of normal weight, but forces exceeding those necessary to retard physeal growth were calculated in the model of the obese child. Our data are consistent with the hypothesis that Blount's disease is primarily the result of the proximal tibial epiphysis responding to physical phenomena.