Although ventricular septal defect is the most common form of congenital heart disease, its epidemiology is not completely understood. The Baltimore-Washington Infant Study, a case-control study of congenital cardiovascular malformations, enrolled 4,390 case infants during the study period 1981–1989. Ventricular septal defect, the most common malformation, was present in 1,465 (33%) of affected infants. The purpose of this report was to describe the epidemiological characteristics of membranous and muscular ventricular septal defects, the most common types of ventricular septal defects. Membranous ventricular septal defects (n=895) and muscular ventricular septal defects (n=429) accounted for 90% of all ventricular defects. During the study, prevalence increased from 0.9 to 2.1 /1000 live births. Associated non-cardiac defects, such as chromosomal defects, genetic syndromes, and defects of other organs were more common among infants with ventricular septal defect than among control infants, and more common among infants with membranous ventricular defect (20.8%) than among those with muscular defects (7.5%). Infants with ventricular septal defects and non-cardiac defects had increased mortality in comparison to infants with isolated ventricular septal defects or controls. Epidemiological information derived from population-based studies provide valuable data for counseling families and may allow for examination of etiologic factors.