The clinical features and morphological findings in 31 Japanese infants with trisomy 18 are presented. The majority were small-for-date infants. There was no sex predominance in our series, as opposed to male:female ratios of 1:3 reported in the literature. The average age at death was greater in females than in males. Cardiovascular anomalies were consistently present; ventricular septar defect and patent ductus arteriosus being the most common malformations. Various other internal malformations including the Arnold-Chiari malformation were observed.