31 adults took part in a randomised, placebo-controlled, double-blind trial of intravenous acyclovir therapy (500 mg/m2 intravenously 3 times daily for 5 days) for acute herpes zoster. Acyclovir reduced pain, decreased erythema, prevented the formation of new lesions, and healed skin faster than did placebo. The duration of viral shedding was also significantly shorter in acyclovir recipients (2 days versus 5 days). However, 6 (35%) of 17 acyclovir recipients had recurrence of pain after the drug was discontinued, and acyclovir did not appear to affect post-herpetic neuralgia. Acyclovir therapy was associated with a transient rise in serum creatinine levels, and may have been related to nausea and vomiting. Intravenous acyclovir was effective therapy for acute herpes zoster but the ideal treatment regimen might be a lower daily dose given for a longer period.