In the 15 years from 1957 to 1971, there were 57 979 births in a circumscribed population in North-West England. The requirements for a valid survey of the incidence of congenital heart disease were largely met. The necropsy rate for all stillbirths and neonatal deaths in the population was 93 per cent. Diagnosis of the type of heart lesion was by necropsy in 50 per cent, cardiac catheterisation in 19 per cent, operation in 5 per cent, and clinical observations only in 26 per cent. The incidence of congenital heart disease was 6-8 per 1000 total births and 5-9 per 1000 live births. The overall incidence has remained unchanged over the 15 years, but there were trends suggesting an increase in the incidence of uncomplicated ventricular septal defect and of endocardial cushion defect and a decrease in the incidence of ventricular septal defect with right ventricular outflow obstruction and of hypoplastic left heart. We have observed seasonal variations in total incidence and in incidence of some common individual lesions.