Seventy-seven patients with first episodes of genital herpes and 111 with recurrent episodes were enrolled in a double-blind trial comparing topical acyclovir with a placebo (polyethylene glycol ointment). Among acyclovir-treated patients with first-episode primary genital herpes, the mean duration of viral shedding (4.1 days) and the time to complete crusting of lesions present at the initiation of therapy (7.1 days) were shorter than among placebo recipients (7.0 and 10.5 days, respectively) (P less than 0.05). Acyclovir-treated patients with recurrent herpes had a shorter duration of viral shedding than placebo recipients (0.95 vs. 1.90 days) (P = 0.03). Among the patients with recurrent herpes, acyclovir reduced the time to crusting of lesions in men but had no effect on the symptoms or healing times in women. Topical acyclovir shortens the duration of viral shedding and accelerates healing of some genital herpes simplex virus infections.