Incidence of congenital heart defects was studied prospectively in all 664,218 children born in 1977 to 1984. Those suspected of having a heart disease were examined at the center specialized in pediatric cardiology. All children who died were autopsied and those in whom a heart defect was proved were included in our series. In total, 4409 infants (6.64/1000 live births) were born with a heart defect in Bohemia. Differences from one year to another were not statistically significant. The incidence of infants with a heart defect was highest in October, lowest in December, June and July (p less than 0.05). Seasonal incidence of individual forms of heart defects differed as well. The seasonal variation was not influenced by the total number of live births. High incidence of congenital heart defects in infants born in October and November coincides with the epidemics of influenza in early pregnancy.