In an ongoing population-based study of congenital cardiovascular malformations in the Baltimore-Washington, DC, area, 1527 affected infants were ascertained from multiple sources during the years 1981 to 1984. Ninety-eight percent were evaluated at a regional pediatric cardiology center. Among the unreferred cases, in which the cardiac defect was diagnosed only at autopsy, most infants died in the first week of life and had associated problems, such as low birth weight, major noncardiac malformations, or other life-threatening illnesses, but a few infants with potentially remediable heart disease escaped clinical detection. Until preventive measures become available, reduction of infant mortality due to congenital cardiovascular malformations will continue to depend on early recognition of signs of serious heart disease in infants and on effective community-wide use of specialized cardiac services.