BACKGROUND: Herpes simplex virus type 2 (HSV-2) infection is common among human immunodeficiency virus (HIV)-infected persons, and HSV reactivation increases plasma and genital HIV-1 levels. We studied HIV-1 levels during HSV suppression in coinfected persons in a placebo-controlled crossover trial. METHODS: Twenty antiretroviral therapy (ART)-naive HIV-1/HSV-2-seropositive men who have sex with men in Lima, Peru, with CD4 cell counts >200 cells/ microL were randomized to receive either valacyclovir at 500 mg twice daily or placebo for 8 weeks, after which they underwent a 2-week washout period and then received the alternative regimen for 8 weeks. Specimens included daily anogenital swabs (for HSV DNA polymerase chain reaction [PCR]), thrice weekly rectal mucosal secretions (for HIV-1 RNA and HSV DNA PCR) obtained by anoscopy, and weekly plasma (for HIV-1 RNA PCR). Outcomes were rectal and plasma HIV-1 RNA levels by treatment arm. RESULTS: HIV-1 was detected in 73% of 844 rectal and 99% of 288 plasma specimens. HSV was detected in 29% and 4% of mucocutaneous specimens obtained during placebo and valacyclovir administration, respectively (P<.001). Valacyclovir resulted in a 0.16 (95% confidence interval [CI], 0.07-0.25; P=.0008; 33% decrease) log(10) copies/mL lower mean within-subject rectal HIV-1 level and a 0.33 (95% CI, 0.23-0.42; P<.0001; 53% decrease) log(10) copies/mL lower plasma HIV-1 level, compared with values for placebo. CONCLUSIONS: Valacyclovir significantly reduces rectal and plasma HIV-1 levels in HIV-1/HSV-2-coinfected men. HSV suppression may provide clinical benefits to persons not receiving highly active ART as well as public health benefits.