Among 2,421 children with Down's syndrome born alive to residents of Massachusetts from 1950 through 1966, approximately one-third had at least one malformation other than the characteristic diagnostic criteria of the syndrome itself. After congenital heart defect, the most common were duodenal obstruction, clubfoot, cataract, imperforate anus, syndactyly or webbed fingers and/or toes, cleft palate and/or lip, and congenital megacolon. Males predominated among children with cataract, syndactyly, and congenital megacolon. Somewhat more females than males had congenital heart defect, although the excess was not significant. The mean maternal age was strikingly low for mongols with cleft palate and/or lip and for males with congenital megacolon. The mean birth weight was significantly lower for mongols with malformations, due primarily to a low birth weight of the children with duodenal obstruction. Most malformations occurred more often in combination with other malformations than expected by chance. The exceptions were duodenal obstruction, congenital megacolon, and imperforate anus. Among 23 mongols with leukemia, 5 died in the neonatal period—a very high age-specific leukemia mortality rate. The sex distribution of cases shifted over time from predominantly males to predominantly females. Leukemic mongols weighed somewhat more at birth than other mongols, although not significantly so. The risk of leukemia among mongols was not related to the presence or absence of congenital malformations.