The boy:girl ratio was calculated for all live births with cardiac malformation in Bohemia (population 6.3 million) from 1977-1984. Complete coverage of all deceased children in Bohemia by necropsy and of all patients with heart disease by our center enabled us to collect reliable data. Of 4409 children born with a heart defect, there were 2296 boys and 2113 girls, a ratio of 1.09:1. In the total population of 664,218 children born during the same period of time, the ratio was 1.06:1. A higher proportion of boys was found with double outlet right ventricle (2.68:1), hypoplastic left heart (2.25:1), transposition of the great arteries (2.11:1), aortic stenosis (1.95:1), pulmonary atresia (1.55:1), tricuspid atresia (1.45:1), coarctation of the aorta (1.30:1), and corrected transposition of the great arteries (1.25:1). There were significantly more girls than boys with persistent ductus arteriosus (1:1.66), Ebstein's anomaly of the tricuspid valve (1:1.57), truncus arteriosus (1:1.22), atrioventricular septal defect (1:1.17), and tetralogy of Fallot (1:1.12). The difference in sex prevalence in the remaining heart defects was less than 10%.