Oral acyclovir at a dose of 800 mg five times daily for seven days was compared with placebo in a randomised double blind trial conducted at three centres in the United Kingdom. The study group comprised 205 elderly immune competent patients suffering from herpes zoster who were entered within 72 hours of the onset of rash. Acyclovir significantly reduced the times to arrest of new lesion formation (p = 0.005), loss of vesicles (p less than 0.001), and full crusting (p = 0.02) in those patients entered within 48 hours of the onset of rash. In addition, there was a significant reduction in pain during treatment with acyclovir as compared with placebo (p = 0.008). Of the patients with severe pain on entry, 40% (10/25) of those treated with acyclovir had no or only mild pain at the end of treatment, whereas in the placebo group all had residual moderate or severe pain (p less than 0.001). No clinically important adverse effects of acyclovir were reported. Oral acyclovir may modify acute herpes zoster and reduce pain.