The incidence of cardiovascular malformations has been studied in a large series of unselected infants. The overall rate was 0.83 per cent, and it varied from 7.7 per cent in infants who were either stillborn or died within the first month (16 of 209), to 0.6 per cent in infants who survived for more than 1 month (see Table I). The accuracy of these incidence rates is discussed, and it is concluded that they are probably somewhat low. Live-born infants weighing less than 2500 gm. at birth who survived for a month or more have a higher incidence of cardiovascular malformations than do those weighing more than 2500 gm. at birth. This difference is of statistical significance. A careful evaluation of etiologic factors in this group of 50 cases revealed the following: a) Maternal age and parity are of some importance as predisposing factors in the causation of cardiovascular malformations. The risk of having an infant with a congenital malformation of the heart or great vessels is greater in the third pregnancy than in the first, especially in women over 30. b) No association between maternal infections early in pregnancy and the occurrence of cardiovascular malformations in the offspring was noted, except for a suggestive bit of evidence that herpes simplex in the first 3 months may be related to the tetrad of Fallot. However, the number of cases of first trimester maternal herpes in this group of 50 cases was not significantly greater than in the entire population, and in all probability the observed association is coincidental. c) Obstetric and medical complications failed to show any correlation with the incidence of vascular anomalies. d) Genetic factors could not be satisfactorily evaluated. The types of malformations encountered are listed, and the methods of diagnosis are discussed. Twenty-three of the fifty cases are still not completely diagnosed, and of these, 20 are alive, and are still under observation. The development of the symptoms in the tetrad of Fallot and in patent ductus arteriosus is discussed. A rather marked difference in maternal age and parity in the two conditions is apparent, but unexplained. Routine RG of the chest in newborn infants was considerably less valuable than clinical evidence in the diagnosis of congenital heart disease. The mortality rates for the group of 50 cases are discussed. Twenty-seven of the fifty patients survived more than 2 years. Associated malformations, involving systems other than the cardiovascular system, occurred in 18 of the 50 cases (36 per cent), and organs derived from the embryonic ectoderm were most frequently affected. Mongolism (4), and cleft palate, cataract, diaphragmatic hernia (3 each) were the most frequently noted anomalies, and the association was thought to be of definite significance in the case of mongolism, and in fatal cases of diaphragmatic hernia.