Acyclovir is a potent antiviral drug, of proven efficacy in the treatment of herpes simplex virus infection. Since this virus has been implicated in the aetiology of Behçet's syndrome, the effect of acyclovir on orogenital ulceration in this condition was determined in a randomized, double-blind, placebo-controlled, crossover trial. Eighteen of 22 patients entering the study completed the trial. Treatment with acyclovir failed to alleviate the frequency and severity of orogenital ulceration or other disease features.