An analysis of 148 radiographs of the tibia during skeletal maturation in patients with achondroplasia was performed. The fibula was always longer than the tibia. The distance between the proximal tibial and fibular physis was less than normal throughout growth. The distance between the distal tibial and fibular physis was normal in childhood and became increasingly greater during growth. Tibia varus increased during growth, and there was a particular increase in distal tibia varus toward the end of growth. The altered growth characteristics of the bones suggest a formulation of cause of bowlegs in achondroplasia.