Prevalence of atrioventricular septal defect in liveborn infants at birth was calculated in Bohemia (population of 6.3 million). Of 664,208 liveborn infants, 4,409 had a congenital malformation of the heart and in 126 of them (0.19/1,000 live births, 2.9% of all heart defects) this was found to be an atrioventricular septal defect. The ratio between those with a common valvar orifice (complete form) versus those with separate right and left atrioventricular valvar orifices (incomplete form) was 1.14:1. The ratio of girls to boys was 1.17:1. Children born in March, July and October were more likely to have the defect (0.26/1,000 live births) than those born in May (0.10/ 1,000 live births). The defect was the eighth most frequent critical congenital cardiac malformation. Eight in each 100,000 liveborn children, and 3.56% of all patients with congenital cardiac disease, are at high risk of dying due to deficient atrioventricular septation, which in 82.5% resulted in the presence of a common atrioventricular valve (complete form). Unconditional death rate was studied in 1,008 liveborn children who died with a cardiac malformation before the age of 15 before the advent of cardiac surgery. Of these, 64 (6.35%) died with an atrioventricular septal defect which was complete in 5.95% and incomplete in 0.40%. All but one of 60 children with the complete form (common atrioventricular valve) died in the first year of life. The last child died between the age of 2 and 5 years. The estimate for natural survival was calculated using our own data on the prevalence and age distribution of mortality. Of all the liveborn children with atrioventricular septal defect, 91% are expected to survive the first month, 81% (78−85) the first trimester, 63% (56−69) six months, and 49% (35−54) the first year. The survival curve remains stable thereafter.