Cytosine arabinoside (cytarabine) was evaluated in a randomized double-blind controlled study for the treatment of localized herpes zoster. Cytarabine was administered subcutaneously in a dose of 50 mg/m-2 body surface area once daily for 4 days, always within 14 days of onset of the first symptom and usually within 7 days. Thirty patients given cytarabine and 30 patients given placebo were well matched with respect to age, sex, and length and severity of presenting rash and pain as well as underlying disease. There was no difference in the rate of disappearance of pain or rash in either treatment group. More patients given cytarabine than patients given placebo had minimal pain and significantly more cytarabine-treated patients had persistence of neurological symptoms at 6 months' follow-up. Acute side effects, though mild, were significantly increased in the cytarabine-treated patients especially with respect to nausea and vomiting and decrease in platelet count. Cytarabine administered in this dose subcutaneously had no beneficial effect and was associated with mild side effects and persistence of neurological symptoms.