Origin of both great vessels from the right ventricle in association with a ventricular septal defect was found in seven of nine autopsied patients with the clinical features of the trisomy-18 syndrome. In four of these patients trisomy-18 was confirmed by chromosomal analyses. This experience indicates that origin of both great vessels from the right ventricle may in fact be an additional component of the trisomy-18 syndrome. Human Genetics, for chromosomal analyses and for consultation.