A study of 491 patients with femur-fibula-ulna (FFU) complex is presented. The term FFU complex has been proposed for cases in which the femur, fibula and/or ulna show defects, which tend to be associated. These cases are usually sporadic. Some rare anomalies of the arms which are present are particularly frequent in FFU complex. These are amelia, peromelia of humerus, humero-radial synostosis and defect of ulna. In our study, 491 patients were investigated for involvement of limb malformations. Our results, showing nearly equal proportions of the most common malformations in four analysed groups (with one, two, three and four limbs affected) supports the hypothesis that even if one arm or one leg only is affected, the cases may still be classifiable as FFU complex. There is a striking asymmetry in presence and in degree. All malformations are more often unilateral than bilateral. Upper limbs are affected more often than lower limbs. The right side and the male sex are preferentially affected. The limb malformations present in the FFU complex are different from those seen in most other types of limb defects, so there is virtually no overlap between FFU and other limb malformations. Some arguments in favour of early somatic mutation as a cause are discussed.