Purpose: A collaborative multicenter doubleblind, placebo-controlled trial of intravenous acyclovir treatment of first-episode genital herpes was performed in order to substantiate previous findings on the efficacy and safety of this drug, to evaluate the influence of parenteral therapy on recurrence frequency, and to obtain further data on the natural history of genital herpes. Patients and methods: Eighty-two patients with first episodes of genital herpes simplex virus (HSV) infection were randomly assigned in a double-blind fashion to treatment with intravenous acyclovir (5 mg/kg every eight hours) or placebo for five days. Before therapy, all lesions in the genital/perineal area and in extragenital sites were cultured. New lesions appearing in both areas after the onset of therapy were cultured separately. Lesions in all groups were cultured until completely healed. Sera were collected from all patients on entry to the study and on Day 21 to determine presence or absence of antibodies to HSV-1 and HSV-2. Time to healing, time to crusting, time to cessation of viral shedding, and appearance of new lesions during therapy were compared for each treatment group. Results: Patients receiving acyclovir experienced a significant reduction in the median duration of pain (4.3 versus 4.8 days, p = 0.019), viral shedding (1.9 versus 8.4 days, p <0.001), and time to healing (8.4 versus 11.5 days, p = 0.02) compared with placebo recipients. These differences were largely attributable to the effect of therapy in the subset of patients with primary disease in whom acyclovir reduced the median duration of pain from 10.6 days to 4.2 days, the median duration of viral shedding from 17.1 days to 1.9 days, and the median time to healing from 14.2 days to 8.3 days. The rate of subsequent recurrence of genital herpes was not altered by acyclovir treatment: 24 of 32 acyclovir recipients (75 percent) experienced one or more recurrences during a mean follow-up of 14 months compared with 19 of 27 placebo recipients (70 percent). Among patients experiencing recurrences, the mean number of recurrences per month among acyclovir recipients was 0.25 compared with 0.19 for patients given placebo. Conclusion: This multicenter trial confirms the efficacy of intravenous acyclovir in the management of first-episode genital herpes, especially in patients with primary infection. However, therapy did not alter the frequency of recurrences. © 1988.