To study the potential effect of prenatal exposure to electromagnetic fields on the occurrence of cogenital urinary tract anomalies (CUTAs) in offspring, we conducted a case-control study in western Washington State. CUTA cases without known chromosomal abnormalities were identified from the Washington Birth Defects Registry. Controls without birth defects were randomly selected from among infants born in five large hospitals in King County, WA. Mothers of cases and controls were interviewed to obtain information on prenatal use of electric blankets, electrically heated water beds, and video display terminals. After adjustment for potential confounders, the risk of CUTAs was found not to be materially associated with these prenatal exposures among all subjects. Among women with a history of subfertility, however, prenatal use of electric blankets was associated with a more than four-fold increase in the risk of CUTAs [adjusted odds ratio (OR) = 4.4; 95% confidence interval (CI) = 0.9-22.7]. The risk was greater if the exposure occurred during the first trimester (adjusted OR = 10.0; 95% CI = 1.2-85.5). The risk also appeared to increase with increasing duration of electric blanket use. Despite small numbers and the potential for recall bias, our study indicates that identifying a susceptible population may be required for detecting adverse reproductive effects of electromagnetic fields.