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/μL 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) log10 copies/mL lower mean within-subject rectal HIV-1 level and a 0.33 (95% CI, 0.23-0.42; P < .0001; 53% decrease) log10 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. Trial registration. ClinicalTrials.gov identifier: NCT00378976. © 2007 by the Infectious Diseases Society of America. All rights reserved.