OBJECTIVE: We examined correlates of HIV infection among injection drug users (IDUs) in Tijuana, Mexico, a city bordering the United States, which is situated on major migration and drug trafficking routes. METHODS: IDUs aged ≥18 years were recruited using respondent-driven sampling. Participants underwent antibody testing for HIV and syphilis and structured interviews. Weighted logistic regression identified correlates of HIV infection. RESULTS: Of 1056 IDUs, the median age was 37 years, 86% were male, and 76% were migrants. HIV prevalence was higher in female participants than in male participants (8% vs. 3%; P = 0.01). Most IDUs testing HIV-positive were previously unaware of their serostatus (93%). IDUs reported injecting with a median of 2 people in the prior 6 months and had been arrested for having injection stigmata (ie, "track-marks") a median of 3 times. Factors independently associated with HIV infection were being female, syphilis titers consistent with active infection, larger numbers of recent injection partners, living in Tijuana for a shorter duration, and being arrested for having track-marks. CONCLUSIONS: Individual, social, and environmental factors were independently associated with HIV infection among IDUs in Tijuana. These findings suggest the need to intervene not solely on individual risk behaviors but on social processes that drive these behaviors, including problematic policing practices. © 2008 Lippincott Williams & Wilkins, Inc.