Acyclovir is a new antiviral agent which is active in vitro and in vivo against a variety of herpesviruses. Two cases are reported in which intravenous administration of acyclovir arrested the progress of disseminated herpes zoster within 24 to 48 hours after the beginning of therapy. There was no evidence of toxicity. Thus, acyclovir appears to be useful in the therapy of herpes zoster, but this requires evaluation.