131 patients with frequently recurring genital herpes were treated for 1 year with reducing doses of oral acyclovir. The time to first recurrence in patients who commenced therapy on 400 mg twice a day was statistically significantly shorter than those on 200 mg four times a day (p less than 0.02) and as the total daily dose and frequency of therapy were lowered so the time to first recurrence was shortened. By the end of 60 days on 200 mg once a day (the lowest daily dose) 56% of patients had recurrences. Patients showed a marked reduction in the frequency of recurrence during therapy (from a mean of 1.1 per 28 days before to 0.11 during treatment, p = 0.0001). After stopping treatment the frequency of recurrences (0.71 per 28 days) was significantly less than the pre-treatment period (p = 0.001). No important side-effects were seen. It is concluded that long-term suppression with acyclovir is safe and effective for patients with recurrent genital herpes.