Health care workers may be occupationally exposed to known and suspected teratogens including viruses, anesthetic gases, sterilants, mercury, and x-radiation. To assess the risk of congenital defects among offspring of health care workers, we analyzed parental occupational histories for 4,915 case babies with congenital defects, registered during the years 1968-1980 by the Metropolitan Atlanta Congenital Defects Program (MACDP) registry, and for 3,027 control babies born without defects during the same period. Offspring of mothers employed in a nursing occupation during the periconceptional period had a modest excess risk of having at least one congenital defect (relative risk [RR] = 1.42; 95% confidence interval [CI] 1.06-1.88); the offspring were at statistically significant increased risk of having anencephaly or spina bifida (RR = 2.00; 95% CI 1.01-4.30), coarctation of the aorta (RR = 2.06; 95% CI 1.10-3.82), genital system defects (RR = 1.61; 95% CI 1.03-2.53), and urinary system defects (RR = 3.43; 95% CI 1.41-8.34). These associations were not confounded by maternal age, education, or alcohol consumption. Offspring of mothers employed in administrative or clerical jobs in the health care industry also had a modest excess risk of defects (RR = 1.35; 95% CI 0.96-1.90), including a statistically significant excess risk of limb defects. We also found associations between neural tube defects and potential exposure to anesthetic gases and to x-radiation, but each association was based on only three case babies of potentially exposed parents. We found no associations between defects and paternal health care employment, except for a few individual defects, and these were based on small numbers of exposed subjects. Only one of five previous studies reviewed found an increased risk of congenital defects among offspring of nurses, but three of the four negative studies had substantially smaller sample sizes than the present study. Detection bias may be a possible explanation for the apparent excess risk of certain defects among offspring of nurses.