The possible effect of environmental factors during pregnancy on the occurrence of atrial septal defect (ASD-secundum) in the offspring was studied in 50 cases and 756 controls. The cases represented all verified ASDs in Finland during 1982-1983. The controls were randomly selected from all infants born during the same period. Case and control mothers were interviewed by midwives using a structured questionnaire approximately three months after delivery. Congenital heart disease was more prevalent among parents of cases than those of controls. Maternal alcohol consumption during the first trimester of pregnancy appeared to double the risk of atrial septal defect (OR = 1.9, CI98 = 1.1 - 3.4). Maternal exposure to chemicals at work during the first trimester was more prevalent among the ASD-group (40.0%) than the control group (26.2%). The risk of ASD was not associated with maternal smoking, or coffee, tea or acetosalicylic acid consumption. Maternal exposure to video display terminals, microwave ovens, organic solvents, anesthetic gases, pesticides or wood preservatives during the first trimester of pregnancy were not associated with the risk of an atrial septal defect. It is concluded that some common physical and chemical exposures during early pregnancy should not necessarily be considered risk factors for atrial septal defect.