The effect of topical acyclovir treatment of first episode genital herpes on the time to first recurrence in a group of 42 patients receiving either acyclovir or placebo was investigated. Topical acyclovir treatment had no effect on time to first recurrence in patients with either first episode HSV-1 or HSV-2 infections. There was no significant difference in the time to first recurrence in patients with either true primary or initial genital infections. However, the time to first recurrence in patients with first episode HSV-2 was significantly shorter than in patients with first episode HSV-1. Acyclovir treatment appeared to have no effect on the development of neutralising antibody in patients with either virus type.