The incidence of certain virus infections in pregnant women was determined serologically in an effort to demonstrate their possible association with congenital anomalies. Significantly more mothers of infants with congenital heart disease were shown to have experienced infection with coxsackieviruses than did matched control women. Coxsackievirus B types 3 and 4 were most frequently associated with malformed infants. Most infections were shown to have occurred during the first trimester of pregnancy. Patient diaries of illness kept by women during pregnancy revealed that half of the infections were completely subclinical. No difference in incidence of infection with other viruses between the anomaly and control groups could be demonstrated.