The association of ectrodactyly and absence of long bones of the upper or lower extremities has been recognized previously in nine families. We report 24 additional individuals in four families who are similarly affected. Two of these families manifest the unusual feature of unilateral preaxial polydactyly of a lower extremity. Data from these four families plus the nine previously reported suggest that ectrodactyly associated with absence of long bones of the upper or lower limbs is a genetically determined disorder, inherited as an autosomal dominant trait with widely variable expression or nonpenetrance.