Twenty-nine patients with first-episode rectal herpes simplex virus infection were enrolled in a double-blind trial of oral acyclovir, 400 mg five times daily, vs placebo treatment. Eighty percent of those receiving acyclovir compared with 25% of placebo recipients no longer had herpes simplex virus isolated from their rectal lesions three days after onset of therapy. The median duration of rectal lesions and viral excretion from rectal lesions (median, five and zero days, respectively) was significantly shorter in patients treated with acyclovir than in placebo-treated patients (14 and 11 days, respectively). Durations of local signs and symptoms of proctitis, such as rectal pain, discharge, and friability, were shorter in acyclovir recipients than in placebo recipients, but these differences were not statistically significant. Daily administration of 2 g of oral acyclovir for ten days alleviates some of the clinical signs of herpes simplex virus rectal infection.