Oral acyclovir, 800 mg five times per day for seven days, was compared with placebo in a randomized, double-blind trial conducted at three centers in the United Kingdom. The study group consisted of 364 elderly immunocompetent patients with herpes zoster who were entered within 72 hours of the onset of rash. Acyclovir significantly reduced the times to last new lesion formation (p less than 0.01), loss of vesicles (p less than 0.01), and full crusting (p = 0.03). No significant hastening of rash healing was seen in those who started therapy later than 48 hours after the onset of rash. There was also a significant reduction pain during treatment with acyclovir (p = 0.02). Acyclovir produced no effects on the frequency or severity of post-herpetic neuralgia. No clinically important adverse effects of acyclovir were reported.