The infantile form of Blount disease is common in the African Negro population of South Africa, although it is very rare in other groups in this country. A series of 110 affected children, investigated in an attempt to elucidate the pathogenesis of this condition, demonstrates an approximately equal sex incidence while geographic distribution is patchy. Bowing of the legs, which is symmetrical in 82% is usually the presenting feature and develops at the onset of ambulation. There is no preceding illness, either local or systemic, and no dietary inadequacy. The children do not differ from their peers in age of walking, weight or range of joint movements. There are no biochemical or hematological abnormalities and no radiographic evidence of involvement of joints other than the knees. Twenty-two of the patients had never been carried in the traditional "pick-a-back" position and it is therefore unlikely that this posture is a significant causative factor. The children had a total of 231 siblings, of whom 10 had bowlegs, while 16 parents had been similarly affected during their own infancy. On this basis, Mendelian dominant or recessive inheritance is unlikely although it is possible that Blount disease is multifactorial in etiology.