BACKGROUND: Herpes simplex virus (HSV) suppressive therapy reduces genital and plasma human immunodeficiency virus type 1 (HIV-1) RNA over periods up to 3 months, but the long-term effect is unknown. METHODS: A total of 484 HIV-1 and HSV type 2 seropositive Tanzanian women aged 16-35 years were enrolled in a randomized placebo-controlled trial of acyclovir administered at a dosage of 400 mg twice daily. Cervico-vaginal lavage and blood samples were collected at 6 months, 12 months, and 24 months for quantification of genital and plasma HIV-1 RNA and genital HSV DNA. Primary outcomes were detection and quantity of cervico-vaginal HIV-1 RNA at 6 months. RESULTS: At 6 months, there was little difference between the acyclovir and placebo arms for cervico-vaginal HIV-1 RNA detection (88 [ 41 .3%] of 213 vs 84 [ 44 .0%] of 191; odds ratio [OR], 0.90; 95% confidence interval [CI], 0.60-1.33), HSV DNA detection (20 [ 9 .4%] of 213 vs 22 [ 11 .5%] of 191; OR, 0.80; 95% CI, 0.42-1.51), genital HIV or HSV loads, or plasma HIV-1 RNA load. Estimated median adherence was 91%. There was a suggestion of an impact on cervico-vaginal HIV-1 RNA detection among women with estimated adherence 90% (OR, 0.74; 95% CI, 0.50-1.09) when data from all 3 visits were included. CONCLUSIONS: Acyclovir administered at a dosage of 400 mg twice daily is unlikely to be a useful long-term intervention to reduce HIV transmission. The lack of effect on HIV may be attributable to suboptimal adherence or treatment regimen.