Employing a clinical approach to the categorization of newborn cardiovascular defects, a list of probabilities has been prepared to aid the pediatrician who is confronted with the common problem of the severely ill newborn heart patient. This study presents the analysis of 155 newborn cardiac autopsies along with 10 surgical successes. These cases were selected from 187 autopsied live-born neonates with major cardiovascular malformations by excluding patients with associated severe non-cardiac disease and major non-cardiac malformations. One-half of the neonatal deaths occurred during the first week with a stepwise decrease in the succeeding weeks. Hypoplastic left heart (rudimentary left ventricle with aortic or mitral stenosis or atresia) was the commonest malformation (37 cases). The second most frequent was complete transposition of the great vessels (25 cases). Most newborns with these two defects died in the first week of life, indicating the urgent need for attention to infants with such anomalies. In contrast, in patients with other malformations, deaths were spread throughout the first month. Neonates who died with combinations of major defects composed the third largest group in this series, emphasizing the difficulties in diagnosis and treatment. Ventricular septal defects, and patent ductus arteriosus were found to be uncommon causes of death or severe distress in the first month of life. As a result of these findings it is evident that a large number of newborns in severe distress from cardiovascular malformations will have defects which are beyond help. However, in many newborn cardiacs, the defects can be relieved or made less severe. Consequently, it is felt by the authors that a newborn infant who is seriously ill due to a suspected cardiac anomaly should be regarded as both a medical emergency and as a potential surgical emergency. Such a patient should be transferred at once to an institution where all facilities and personnel are available for complete investigation and for surgery. Cardiac catheterization and angiocardiography should be performed as soon as the pediatric cardiologist feels that these tests are indicated, often within a matter of hours. This will frequently be in the first days or even in the first twenty-four hours of life. The techniques, problems, and hazards involved in such studies have been presented in detail by Rowe. A maximum of information should be sought inasmuch as the additional risk is usually more than compensated for by the increased accuracy and completeness of the diagnosis. Admittedly, the risk of cardiac catheterization and selective angiocardiography is high in a seriously ill newborn cardiac, perhaps of the order of 5 to 10%, but if this hazard is balanced against the extremely high one of death in a matter of hours or days from the underlying disease, there will be little hesitation about proceeding with the investigation. In certain instances it may be advisable to perform surgery without prior cardiac catheterization and angiocardiography. Such unusual instances involve severely ill newborns in whom the clinical diagnosis seems clear and for whom surgery may be lifesaving. The outstanding example of this is the infant who has the classical findings of pulmonic stenosis with an intact ventricular septum, and who appears to be dying in heart failure. Here the risk of cardiac catheterization may outweigh that of surgery. Other instances include occasional critically ill patients with typical findings of coarctation of the aorta, tetralogy of Fallot, complete transposition of the great vessels, vascular rings, and the rare patent ductus arteriosus in whom the clinical diagnosis is obvious. The authors believe that an aggressive approach to the diagnosis and treatment of the distressed newborn cardiac will substantially improve his chances for survival.