Injection drug users (IDUs) are at high risk for hepatitis B virus (HBV); however, they often do not receive preventive vaccination. IDUs who use mobile health care services linked to a syringe exchange program in New Haven were routinely screened for HBV, hepatitis C virus, and syphilis. Individuals without prior exposure to HBV were offered three-part vaccination series. Of the 212 IDUs screened for HBV infection, 134 (63%) were eligible (negative for HBV surface and core anti-bodies) for vaccination and 10 (4.7%) had evidence of chronic HBV infection. Compared to those with previous exposure to HBV, vaccine-eligible patients were significantly more likely to be younger and use heroin and less likely to be black, home-less, daily injectors, and cocaine users. Of the 134 vaccine-eligible subjects, 103 (77%) and 89 (66%) completed two and three vaccinations, respectively. Correlates of completing all three vaccinations included older age (OR = 1.06, 95% CI = 1.04-1.07), injecting daily (OR = 2.12, 95% CI = 1.36-6.73), and being homeless (OR = 1.98, 95% CI = 1.14-12.27). These results suggest that IDUs remain at high risk for acquiring HBV infection. Programs that link health care to a syringe exchange program are effective ways to provide preventive health care services to IDUs, particularly HBV vaccination. Trust engendered by and mutual respect afforded by such programs result in repeated encounters by active IDUs over time.