Comparative data on fetal and neonatal deaths following maternal mumps, rubella, hepatitis, chicken pox and measles were obtained in a prospective study in New York City from 1957 to 1964, inclusive. The evidence pointed to an increase in early fetal death rate after rubella and mumps and an increase in perinatal mortality after rubella and hepatitis. A significant increase in these rates was not demonstrable for chicken pox and measles in the selected population studied and under the conditions of the present study. The lethal effects of maternal virus diseases were demonstrable in cases of mumps and rubella occurring in the early weeks of gestation and in cases of hepatitis occurring in the late weeks of pregnancy. Fetal death was attributable to severity of maternal disease in hepatitis and to early infection of the fetus in rubella. Other factors related to gonadal infection and to placental and hormonal changes early in pregnancy may be influential in the lethal effect of mumps. Maternal and fetal death occurred in single cases of chicken-pox pneumonia and hepatitis.