Information on 5249 children with congenital heart disease (CHD) in the age group 0-15 years was collected from all paediatric and cardiological departments in Denmark and, furthermore, from death certificates. The mean prevalence for CHD in Denmark was 6.1% during 11 years (1963-73); during the years 1972-73 it was 7.0%. The mean age at detection was 25.86 months; 36% were diagnosed within the first month of life and 63% within the first year. The sex distribution was; 52%, female, 48% male. No seasonal variation was found in the months of birth for individual cardiac anomalies. The largest group of CHD was ventricular septal defect, 24.0%, followed by persistent ductus arteriosus, 12.6%; atrial septal defect, 9.2%; coarctation of the aorta, 7.0%; pulmonary stenosis, 6.0%; and tetralogy of Fallot, 5.8%. The cumulated lethality for all 5249 children ws 25% at 1 month, 42% after 5 years, and 48% at the age of 15 years. CHD was found in 135 cases among 5835 siblings (2.3%).