The efficacy of oral acyclovir 400 mg twice daily in suppressing frequently relapsing genital herpes simplex was evaluated in an open multicenter study. Seventy-one patients were treated for 12 months. During treatment, 73% of the patients were completely free of symptoms when taking the tablets continuously, and another 14% had mild symptoms such as erythema and/or itching at single occasions. An accidental treatment interruption for 2-4 days led to mild but definite herpes episodes within a few days in 5 otherwise symptom free patients. Definite herpes episodes despite acyclovir medication occurred in 3 cases (4%). No noteworthy side effects were recorded during the acyclovir treatment. After withdrawal of acyclovir, herpes relapsed within 1-4 weeks in 69% of the patients. The frequency of relapses during the following months was reported to be equal to that before the treatment period in most of the patients. Acyclovir susceptibility of the isolated herpes simplex virus (HSV) strains did not change during treatment. The mean titres of antibodies against HSV type-common glycoprotein antigen, HSV-2 type-specific antigen and varicella zoster virus antigens decreased significantly during treatment with acyclovir.