Intravenous acyclovir was tested both prophylactically and therapeutically in herpes infections in 48 patients undergoing bone marrow transplantation. Pretransplant prophylaxis only partly eliminated reactivated infections. Treatment of established mucocutaneous infections was usually successful, also in relapsing cases, when initiated within hours of the appearance of lesions. Extensive lesions, superinfections and dissemination could thus be prevented in this group of severely immunocompromised patients.