Rigorous clinical trials have established that both acyclovir and vidarabine favourably alter the clinical course of herpes zoster and chicken-pox in immunocompromised patients. In one comparative study, acyclovir was shown to be superior to vidarabine for zoster in bone marrow transplant recipients. These data, plus the fact that acyclovir is easier to administer than vidarabine, and perhaps less toxic, have made intravenous acyclovir the recognized drug of choice for treatment of herpes zoster in immunocompromised patients. Acyclovir sodium sterile powder received Federal Drug Administration (FDA) approval for this indication in 1990 in the United States. Since complications of zoster occur in only a minority of immunocompromised patients, most physicians would prefer to initiate therapy with an orally-administered drug and avoid the cost and inconvenience of hospitalization. Future studies will compare the efficacy and safety of orally administered bromovinyl arabinosyl uracil and acyclovir in treatment of varicella-zoster virus infections. © 1991.