Twenty-seven bone marrow transplant patients who developed localized herpes zoster were treated with acyclovir in a randomized study comparing oral and intravenous drug administration. Fourteen patients received oral and 13 patients received intravenous treatment. None of the patients developed disseminated disease. No differences were found between the treatment groups in the number of days that new lesions continued to develop, in number of days with pain, or in the number of days from start of treatment until all lesions were crusted over. We suggest that oral acyclovir may be as effective as intravenous acyclovir in the treatment of localized herpes zoster after bone marrow transplantation.