Seventy patients with 91 congenital short femora are classified. Deformities resulting maternal Thalidomide treatment are compared with those where Thalidomide was not involved and genetic and epidemiological factors investigated in 50 patients. No essential anatomical difference was found between the two groups of femora but the whole complex of abnormalities differed: the Thalidomide group showed femur-tibia-radius anomalies while the non-Thalidomide garoup had femur-fibula-ulna anomalies, indicating either different aetiological factors or different timing of the insult to the foetus. Some differences between congenital coxa vara and congenital short femur associated with coxa vara are mentioned. Simple hypoplasia of the femur may possibly have a multifactorial genetic background since it is associated with other minor abnormalities of the limbs in these families, whereas environmental factors only are associated with the more severe femoral defects.