The design of the Baltimore-Washington Infant Study was that of a case-control study. All infants under one year of age with confirmed diagnoses of congenital heart disease were eligible for inclusion if they were residents of the study area which encompassed 53 area hospitals in Maryland, the District of Columbia and five counties in Virginia. Case enrollment was done through five pediatric cardiology centers and through a periodic search of the obstetrics and neonatal and pathology logs of the participating hospitals. Control selection was by random numbers and all resident births were eligible as controls except for those with congenital heart disease. Mothers of cases and controls were interviewed at home for demographic information, and information on maternal health, maternal medication, reproductive history, lifestyle, environmental exposures in the home, occupation, and agents transmitted to the mother by the father. Data were collected on the characteristics, drug use, habits, and occupations of the fathers. Vital records and birth certificates were abstracted for all cases and controls. Death certificates were also abstracted. Variables including drugs, lifestyle and home exposures, and occupation, were screened to identify which single factors were most importantly related to congenital heart disease.