We assessed the relation between febrile illness during pregnancy and cardiac defects in the offspring in a population-based case-control study in metropolitan Atlanta. Case infants (905) with cardiac defects were actively ascertained from multiple sources. Control infants (3,029) were infants without birth defects who were selected from birth certificates by stratified random sampling. We compared those whose mothers reported febrile illness from 1 month before pregnancy through the third month of pregnancy with those whose mothers reported no illness during the same period. Febrile illness was positively associated with the occurrence of heart defects in the offspring (odds ratio [OR] = 1.8; 95% confidence interval = 1.4-2.4). When influenzalike illness was the reported febrile illness, the OR was 2.1 (95% confidence interval = 0.8-5.5). The association with febrile illness was strongest for tricuspid atresia (OR = 5.2), left obstructive defects (OR = 2.7), transposition of the great arteries (OR = 1.9), and ventricular septal defects (OR = 1.8). These ORs were generally lower among mothers who used multivitamins during the periconceptional period.