Neonatal herpes affects about 1 in 15,000 newborns and the prognosis for disseminated disease with encephalitis is poor. We investigated whether acyclovir prophylaxis in late pregnancy effectively reduces the risk of viral shedding and, hence, of mother-to-child transmission at delivery. A prospective study was conducted. Pregnant women who had at least one episode of genital herpes during pregnancy were randomly assigned to two groups: group 1 (n=167) received oral acyclovir from 36 weeks of gestation to term; group 2 (n=121) received no treatment. Group 3 (n=201) comprised women not given prophylaxis who had a history of genital herpes, but no active episodes during pregnancy. No specific instruction were set up for obstetrical management except for cesarean section in case of a suspected herpes lesion at the time of labor. The rate of Cesarean section was 8.4% in group 1, 16.5% in group 2, and 9.9% in group 3 (p<0.001). 75% of cesareans in group 2 and 10% in group 3 were done for genital herpes. Percentage of viral shedding was, respectively, 0% (group1), 5% (group2), and 0.5%(group3) (p<0.05). These findings underline the value of antiviral prophylaxis in late pregnancy for women with a known history of genital herpes. Such prophylaxis only partly prevents neonatal herpes infection, because it is not applicable to patients with no known clinical history but may excrete the virus.