The possible effects of working with video display terminals (VDTs) during pregnancy on the occurrence of cardiovascular malformations in the offspring was studied in 500 cases and 1055 controls. The cases represented all registered cardiovascular malformations reported in Finland during 1982-84, excluding those with chromosomal anomaly or known genetic syndrome. The controls were randomly selected from all babies born during the same period. Both the case and control mothers were interviewed by midwives using a structured questionnaire approximately three months after delivery. In this interview the mother's occupation, job description and employer during the first trimester were noted, as were large number of other exposures to chemical and physical factors. An industrial hygienist examined all these records for information indicating exposure to VDTs, unaware of case/control status. Work with VDTs during the first trimester of pregnancy was ascertained for 30 case mothers (6.0%, 30/500) and 53 control mothers (5.0%, 53/1055). In logistic regression analysis maternal exposure to VDTs for at least 20h a week during the first trimester of pregnancy showed a point estimate of odds ratio of 1.4 with 95% confidence limits of 0.5 and 3.8, when adjusted for age and alcohol use. Maternal exposure to VDTs was not associated with indicators of fetal growth such as birthweight, placental weight or length of gestation.