The HIV epidemic in Vietnam is concentrated primarily among injecting drug users (IDUs). To prevent HIV-1 superinfection and to develop effective HIV prevention programs, data are needed to understand the characteristics of high-risk HIV-positive IDUs. In 2003 , we conducted a community-based cross-sectional study among predominately male, out-of-treatment IDUs, aged 18?45, in the Bac Ninh Province, Vietnam. Among 299 male participants, 42.8% were HIV-positive, and among those, 96.9% did not know their status prior to the study. Furthermore, 32% were HIV-positive and had high HIV behavioral risk (having unprotected sex or having shared injecting equipment in the past 6 months). Injecting for ?3 years, younger age, and pooling money to buy drugs were independently associated with being at high risk for transmitting HIV. IDUs who purchased more than one syringe at a time were less likely to have high HIV behavioral risk. Structural interventions that increase syringe accessibility may be effective in reducing HIV risk behavior among HIV-positive IDUs. Study limitations are noted in the article.