A total of 113 patients with recurrent genital herpes were enrolled in a multicentre cross-over study. Two consecutive recurrences were treated with either oral acyclovir 200 mg or matching placebo five times daily for five days. Acyclovir significantly reduced the time to crusting and healing and the duration of new lesions and symptoms. Acyclovir was not found to have any clinically significant effect on the haematological or biochemical parameters measured, or to have caused adverse events and was well tolerated. Early self-initiated treatment may abort a recurrent attack of genital herpes without progression to the ulcerative or crusting stage.