SummaryA statistical study was carried out on 377 families containing children with congenital heart disease, seen and diagnosed in the Cardiac Department of Guy's Hospital. The patients were divided into three main clinical groups: (1) those with Fallot's tetralogy; (2), (3) and (4) those with other forms of cyanotic heart disease; and (5) and (6) those with acyanotic heart disease. Five cases of mongolism and two following maternal rubella were excluded.The following conclusions were reached:I. There was an excess of affected children with Fallot's tetralogy born between maternal ages 40 and 44 years; otherwise no effect of maternal age was evident. Paternal age and birth rank had no effect.II. Although the seasonal incidence of congenital heart disease was the same as in the general population, there was a significant difference of seasonal incidence between males and females.III. There was a significant excess of males among the cases of Fallot's tetralogy, coarctation of the aorta, transposition of the great vessels, and, especially, congenital aortic stenosis. The aorta of the male appears to be more vulnerable to congenital and acquired defects than that of the female.IV. The incidence of congenital heart disease among the sibs born after the propositus was about twenty times the incidence in the general population (2 per 100 as against 1 per 1000 at the age of 10 years).V. No association was observed between twinning and congenital heart disease.VI. There was no excess of stillbirths, miscarriages or abortions, nor was there any concentration of disturbed pregnancies.VII. Search was made for a period of relative sterility preceding or following the birth of the affected child but convincing evidence of this was not found.VIII. There were twelve examples of associated malformations in the 377 propositi, cleft palate figuring three times, congenital club foot, mental defect and hemivertebra twice each. The incidence of malformations, other than congenital heart disease, in the sibs of the propositi was 2 %.IX. The incidence of congenital heart disease in relatives other than sibs was investigated.X. The incidence of first‐cousin marriages among parents was 0.8%, a figure not significantly different from what could be expected in a hospital population (0.4%) yet identical with the percentage to be expected on the hypothesis that congenital heart disease results from simple autosomal recessive inheritance.XI. Maternal infections are of importance only in rare cases.XII. There was no evidence of limitation of family siz