In several studies investigators have suggested that maternal use of exogenous sex hormones during early pregnancy may be associated with various congenital malformations. A group of malformations, the VACTERL (vertebral, anal, cardiac, trachea, esophageal, renal, limb-acronym) association, has been statistically associated with maternal exposure to exogenous sex hormones during the first trimester of pregnancy. The VACTERL association is a nonrandom group of major malformations that occur together more often than would be expected on the basis of chance. To assess this association, we conducted a case-control study of first-trimester exposure to sex hormones among mothers of 34 infants with the VACTERL association and of 1,024 comparison infants with one or more of ten major malformations or Down syndrome. The study subjects were malformed infants born between July 1970 and June 1979 and registered in a population-based birth defects registry. Information concerning the use of exogenous sex hormones during pregnancy was obtained by systematically interviewing the mothers of the malformed infants. Most of the mothers were interviewed within 6 months of their children's births. Each mother was interviewed within a year of her child's birth. We found an odds ratio of 0.98 (90% confidence limits 0.40, 2.38) for the relationship between VACTERL association and use of any sex hormone in the first trimester of pregnancy. Our study had adequate statistical power to detect a true relative risk of 2.8 or greater.