Acyclovir, a new antiviral agent, was compared to a placebo in a randomized double-blind trial of treatment for culture-proven herpes simplex virus infection after marrow transplantation. Patients received either intravenous acyclovir at 750 mg/m2 body surface area per day or a placebo for 7 days. Thirteen of 17 patients given acyclovir had a beneficial response as compared with two of 17 given the placebo (p less than 0.01). The duration of positive cultures was shorter among acyclovir recipients (3 versus 17 days, p less than 0.00005). Also shorter were the median days to resolution of pain (10 versus 16 days, p = 0.03), to crusting of lesions (7 versus 14 days, p = 0.01), and to total healing (14 versus 28 days, p = 0.03). No acyclovir toxicity was observed. Recurrent infection was common. Acyclovir provided significant antiviral and clinical efficacy without toxicity in highly immunosuppressed patients but had no effect on virus latency.