In this randomized, double-blind, multicenter, acyclovir-controlled study, the efficacy and safety of famciclovir were evaluated for the treatment of herpes zoster in patients who were immunocompromised following bone marrow or solid organ transplantation or oncology treatment. A total of 148 patients, 12 years or older with clinical evidence of localized herpes zoster, received either oral famciclovir, 500 mg three times daily, or acyclovir, 800 mg five times daily, for 10 days. Famciclovir was equivalent to acyclovir with respect to the numbers of patients reporting new lesion formation while on therapy (77% vs. 73%, respectively). There were no significant differences between the groups in the time to cessation of new lesion formation, full crusting, complete healing of lesions, or loss of acute phase pain. Treatment with famciclovir was well tolerated, with a safety profile comparable to that of acyclovir. Thus oral famciclovir is a convenient, effective, and well-tolerated regimen for immunocompromised patients with herpes zoster.