Epidemiologic studies of hepatitis C virus (HCV) infection are unusual in developing countries, especially Thailand. We evaluated the prevalence and risk factors for HCV among military conscripts, including a sample of 5,246 men (1:30 sample), and human immunodeficiency virus (HIV)-1 positive men (N = 500) between 2005 and 2008. The HCV prevalence was 2.2% in the sampled group and 8.4% in HIV-1 sero-positives. Among the sampled group, HIV-1 infection, injection drug use (IDU) history, and unsafe injections were associated with HCV infection; adjusted prevalence rate ratios [RRs; 95% confidence intervals (CIs)] were 3.7 (1.04-12.77), 1.9 (1.04-3.54), and 1.8 (1.02-3.11), respectively. Among HIV-1 sero-positives, an IDU history and residence in southern Thailand were associated with HCV prevalence; adjusted RRs (95% CIs) were 3.5 (1.71-7.24) and 2.6 (1.18-5.61), respectively. Public health measures to prevent HCV in Thailand should focus on reducing injection drug use and other exposures to unsafe injections among young Thai men. Copyright © 2010 by The American Society of Tropical Medicine and Hygiene.