The suppressive action of acyclovir and inosine pranobex was compared in a randomised double blind controlled trial in patients with frequently recurring genital herpes. Fourteen patients received acyclovir and 17 inosine pranobex. Treatment continued for 12 weeks. The time to the first recurrence was significantly longer and the frequency of recurrences significantly less in the recipients of acyclovir. No important side effects were noted. It is concluded that acyclovir is the treatment of choice to suppress often recurring genital herpes.