Certain clinical and morphologic observations are described in 73 necropsy patients with aortic valve atresia. The mean age at death was 5 days; 80 percent died during the first week of life, and 70 percent were boys. Of the 73 patients, 69 (95 percent) had a hypoplastic left ventricle with intact ventricular septum and either an atretic (25 patients) or hypoplastic (44 patients) mitral valve. The other four patients had a well developed left ventricle with one or more defects in the ventricular septum and either an atretic (one patient) or well developed (three patients) mitral valve. Review of previous reports on aortic valve atresia disclosed that a well developed left ventricle or ventricular septal defect in association with absence of the aortic valve was extremely rare. A new classification for aortic valve atresia is presented based on the status of the ventricular septum, which in turn appears to determine the size of the left ventricular cavity. The predilection for male subjects for all types of aortic valve disease, including atresia, is emphasized.