BACKGROUND: HIV prevalence in Vietnam is currently concentrated among injection drug users (IDUs). The extent to which this core risk group represents a potential for broader HIV transmission to the general population is currently unknown. METHODS: A community-based cross-sectional study among IDUs in Vietnam assessed sexually transmitted disease (STD) prevalence and behavioral risk factors. Qualitative interview data enhanced quantitative findings. RESULTS: The prevalence of any STDs among 272 IDUs was 30% (chlamydia, 9%; herpes simplex virus type 2 [HSV-2], 22%; gonorrhea, 0%; and syphilis, 1%). Part-time work or unemployment (odds ratio [OR] = 2.74, 95% confidence interval [CI]: 1.1 to 6.9), sex with > or =2 sex workers in the past year (OR = 4.9, 95% CI: 1.91 to 12.6), having ever smoked heroin (OR = 4.5, 95% CI: 1.1 to 18.3), and injecting less frequently than daily (OR = 3.9, 95% CI: 1.43 to 10.6) were independently associated with chlamydial infection. Urban residency (OR = 4.0, 95% CI: 1.4 to 11.0) and daily injecting (OR = 2.2, 95% CI: 1.1 to 4.4) were independently associated with HSV-2. Odds of HSV-2 among older (> or =28 years of age) IDUs who had sex with <2 sex workers in the past year was higher than among younger IDUs who had sex with more sex workers (OR = 6.4, 95% CI: 2.1 to 18.4). CONCLUSIONS: High STD prevalence and high-risk sexual and parenteral behaviors among IDUs indicate the potential for HIV/STD transmission to the general Vietnamese population.