The incidence of congenital heart disease (C.H.D.) in Liverpool from 1960 to 1969 inclusive has been determined from the Liverpool Congenital Abnormalities Registry with a follow-up period of 3 to 12 years. The incidence is 6-6 per 1000 total births and this probably represents a very small degree of under-reporting. There is no consistent seasonal variation in the incidence of any of the main congenital heart lesions. In general, infants with C.H.D. tend to be of lower birth weight and born after shorter gestation than controls. This is most conspicuous with patent ductus arteriosus (P.D.A.). Females preponderate in P.D.A. and males in transposition. There is probably also a male preponderance in coarctation and aortic stenosis. Fallot's tetralogy is associated with increased maternal age and parity. Pregnancies leading to the birth of a baby with C.H.D. are complicated by threatened abortion more frequently than are controls. The concordance rate for C.H.D. in twins is low. Monozygotic twins are more liable to C.H.D. than are dizygotic twins. The incidence of C.H.D. in the siblings of affected propositi is 2-3 times that expected. Affected sibs often have the same lesion. About 20 per cent of infants with C.H.D. have associated major defects notably monogolism and defects of the alimentary, skeletal, genito-urinary and nervous systems. These are responsible for the early death of about one quarter of all infants born with C.H.D. The data presented here suggest that environmental rather than genetic factors are predominantly responsible for congenital heart disease.