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We studied 35 otherwise healthy adults with frequently recurring genital herpes (greater than or equal to 1 episode per month), in a double-blind trial comparing oral acyclovir with placebo capsules for suppression of recurrent infection. The patients were treated for 125 days unless herpes recurred. Among 32 evaluable patients, there were significantly fewer recurrences during acyclovir treatment (4 of 16) than during placebo treatment (16 of 16, P less than 0.001). The mean duration of therapy was significantly longer for patients receiving acyclovir than for those receiving placebo (114.9 vs. 24.8 days, P less than 0.001). Of 19 patients who had recurrences in the blind trial, only 2 had recurrences when given acyclovir in a second, open-study phase. All patients had recurrences after completing acyclovir treatment. The therapy was well tolerated, with minimal gastrointestinal upset and one hypersensitivity reaction. Studies of the viral isolates demonstrated that lesions developing in patients receiving acyclovir contained drug-resistant virus. Later recurrences in these patients were associated with drug-sensitive virus. We conclude that oral acyclovir suppresses genital herpes in patients with frequent recurrences, but the potential for problems with drug resistance and the long-term safety need to be more fully explored.