Sixty immunocompetent patients with herpes zoster of various dermatomes were treated 5 times a day for 5 days with either acyclovir at a dose of 400 mg or placebo. Acyclovir was shown to reduce significantly the time to full crusting (P = 0.02). There were also trends in favour of acyclovir for time to first dry vesicle and time to first day without macules or papules, but these were not statistically significant. There were no differences between the groups in the occurrence of adverse events or postherpetic neuralgia.