A subunit antigenoid vaccine, Ac NFU1 (S-) MRC 5, was used in patients who had had a clinical episode of herpes genitalis. The rate of recurrence was compared with that in unvaccinated patients to determine the efficacy of vaccination in preventing recurrence and spread of the virus in the community. Seven of 22 (31%) vaccinated patients had eight recurrences after the initial clinical episode; in contrast there were 51 recurrences in 17 of 20 (85%) unvaccinated patients. Although further studies are needed, the results indicate that the vaccine may prevent recurrent episodes of herpes genitalis and thereby reduce the dissemination of this virus in the population.