We investigated the consequences of maternal infection with Varicella Zoster virus in a prospective study of 43 pregnancies complicated by varicella and 14 pregnancies complicated by herpes zoster. Nine of 43 pregnant women with varicella had associated morbidity — pneumonia (4 women), death (1), premature labor (4 of 42), premature delivery (2 of 42), and herpes zoster (1). Intrauterine varicella infection was identified on the basis of clinical evidence (anomalies characteristic of the congenital varicella syndrome, acute varicella at birth, or herpes zoster in infancy) or immunologic evidence (IgM antibody to Varicella Zoster in the neonatal period, persistent IgG antibody to Varicella Zoster at one to two years of age, or in vitro lymphocyte proliferation in response to varicella–zoster virus antigen). The congenital varicella syndrome occurred in 1 of 11 infants of women with first-trimester varicella. Immunologic evidence of intrauterine varicella infection was present in 7 of 33 infants tested; 4 of these infants were asymptomatic. According to clinical or immunologic criteria, 8 of 33 infants had evidence of intrauterine varicella infection. These observations show that varicella during pregnancy was associated with maternal morbidity and evidence of fetal infection, but that herpes zoster was not. (N Engl J Med 1986; 314:1542–6.)