46 patients with acute herpes zoster ophthalmicus of less than 72 hours duration were recruited into a placebo controlled trial to assess the efficacy of oral acyclovir, 800 mg 5 times daily, in preventing or modifying ocular complications and pain. Fewer acyclovir recipients developed intraocular complications and these were less severe but neither difference was statistically significant. However, active ocular disease was significantly less common in the acyclovir group (p = 0.01) at 6 months. Pain was significantly less severe in the acyclovir group between 2 and 6 months. The proportion of patients with pain scores greater than 0 was significantly lower in the acyclovir group between 2 and 3 months. Oral acyclovir appears to modify the disease process in herpes zoster ophthalmicus, to reduce the severity and incidence of postherpetic pain and especially to protect against long-term ocular complications.