Infection with Kaposi's sarcoma-associated herpesvirus (KSHV) is common among men who have sex with men (MSM). To determine correlates of infection, 578 human immunodeficiency virus (HIV)-negative MSM were assessed by serologic assays, questionnaires, and physical examinations. At baseline, 76 (16%) of 474 participants were KSHV seropositive. Prevalent KSHV infection was significantly associated with hepatitis A (odds ratio [OR], 3.3; 95% confidence interval [CI], 1.5-7.5), hepatitis B seropositivity (OR, 2.6; 95% CI, 1.4-4.8), herpes simplex virus (HSV)-2 (OR, 2.4; 95% CI, 1.3-4.4), and >4 male partners in the previous 6 months (OR, 1.9; 95% CI, 1.1-3.2). Fifteen KSHV seroconversions (4%) were observed for an incidence of 3.8/100 person-years, similar to HSV-1 incidence in this cohort and more frequent than incidence of HIV and HSV-2. Reporting > or =1 HIV-positive partner (OR, 5.9; 95% CI, 1.8-19.3), amyl nitrite use (OR, 7.0; 95% CI, 2.1-23.0), and lymphadenopathy in the past 6 months (OR, 7.7; 95% CI, 1.9-31.0) correlated with KSHV seroconversion.