Sixty-four patients received systemic alpha-interferon (10 million units subcutaneously daily) and 63 received systemic acyclovir (5 mg/kg body weight intravenously thrice daily) in a randomized study of acute herpes zoster. Start of healing, complete healing, development of new skin lesions in the primarily affected and in other dermatomes, and degree and duration of pain were evaluated. Both drugs proved equally clinically efficient without statistically different findings between the two groups; herpes zoster neuralgia was not prevented by either interferon or acyclovir therapy. Minor clinical side effects occurred slightly more frequently during interferon treatment and included fever and nausea. Transient and moderate leukopenia was observed in nearly all patients in the interferon group.