The possible effect of genetic and environmental factors during pregnancy on the occurrence of ventricular septal defect (VSD) in the offspring was studied in 150 cases and 756 controls. The cases represented all verified VSDs in Finland during 1982-1983. The controls were randomly selected from all babies born during the same period. Case and control mothers were interviewed by midwives approximately three months after delivery using a structured questionnaire. Congenital heart disease was more prevalent among parents of cases than those of controls. Maternal alcohol consumption during the first trimester of pregnancy was more common among the mothers of VSD infants (47.0%) than among those of controls (38.0%, P less than 0.05). Exposure to organic solvents at work showed in logistic regression analysis an adjusted relative odds ratio of 1.8 (95% confidence interval 1.0-3.4). The risk of VSD was not associated with any of the maternal habits monitored, e.g. smoking, or coffee, tea, cola, acetosalicylic acid or diazepam consumption. Whether the mother was employed during the first trimester of pregnancy, and her exposure to anesthetic gases, disinfectants, pecticides, wood preservatives or video display terminals were not factors associated with the risk of ventricular septal defect.