This study demonstrates seasonal variations of birth dates in children with congenital valvular heart disease and in adults dying from valvular heart disease. The findings are based on the 1989-98 Swiss EUROCAT data, and on 1969-94 Swiss mortality records. Seasonality was tested with aggregated monthly data using Edwards' procedure. Both data sets showed excesses between December and March, consistent in different forms of valvular disease and in both sexes. Despite the decline of rheumatic heart disease, risk factors causing season of birth effects remain relevant for congenital anomalies.