This multicentre, double-blind, double-dummy, randomised study was undertaken to compare the efficacy and tolerability of famciclovir administered at 250 mg, 500 mg and 750 mg three times daily with acyclovir 800 mg five times daily for the treatment of acute uncomplicated herpes zoster in immunocompetent adults. A total of 545 patients participated in this trial. Treatment was initiated within 72 h of the onset of the zoster rash and was continued for seven days. When treatment was initiated within 72 h, famciclovir was found to be as effective as acyclovir at all dose levels for cutaneous lesion healing as demonstrated by the median times to full crusting, cessation of new lesion formation, loss of vesicles and loss of crusts; time to loss of acute pain was comparable in patients receiving famciclovir and acyclovir. Time to resolution of zoster-associated pain, however, occured at a significantly faster rate in patients treated with famciclovir within 48 h of rash onset compared with acyclovir treatment. Famciclovir was well tolerated with a safety profile comparable to that of acyclovir. Gastrointestinal disturbances and headache were the most common adverse experiences in all treatment groups. In conclusion, famciclovir, administered less frequently and at lower unit doses than acyclovir, is an effective treatment for patients with uncomplicated herpes zoster.