We report the anatomical variations of the limbs in eight infants with the trisomy-18 syndrome that were dissected and studied in detail. In each case, the upper limbs showed defects which further define the specific influence of this aneuploidy on the development of its preaxial (radial) component, and the tendency towards reduction defects. Abnormalities included muscle variations concentrated along the radial margin of the forearm and hand, the absence of the definitive musculocutaneous nerve in all of the limbs, and reductions of the radial artery in four of the bodies. Pathogenetic mechanisms explaining the observed defects are discussed, and include: 1) a defect in peripheral nerve development; or 2) tissue necrosis. The characteristic flexion deformities of the fingers seem to be due to a displacement of the tendons of extensors digitorum and digiti minimi. The lower limbs did not show a consistent pattern of defects, except for the absence of some muscles (psoas minor, the tendon of flexor digitorum brevis to digit V), and the presence of several supernumerary muscles. These variations are discussed as possible nonspecific effects of 18-trisomy on development. The additional anatomical data from this and the first paper in this series [Bersu and Ramirez-Castro, 1977] provide a more detailed picture of the trisomy-18 phenotype which may be useful in corroborating an unconfirmed clinical diagnosis of the syndrome.