To assess whether patterns of HIV transmission have changed in Thailand, we tested for antibody to hepatitis C virus (HCV) as a marker for parenterally acquired infection among HIV-infected and uninfected young Thai men. Antibody to HCV was present in 49.5% of HIV-infected men and 2.2% among uninfected men. These data suggest that a significant number of HIV infections among young men in Thailand may be associated with injection drug use.