A total of 125 heroin users were recruited from a detention center and two psychiatric hospitals in northern Taiwan during 2006 in order to investigate the prevalence and correlates of blood-borne infections among heroin users. The seroprevalence rates of the human immunodeficiency virus (HIV), hepatitis C virus (HCV), HBV, HDV, and syphilis were 15.2%, 74.4%, 15.2%, 6.4%, and 8%, respectively. Injection risk behaviors were associated with HIV, HCV, and syphilis infections, but not with HBV infections. Meanwhile, HCV and HBV infections were correlated with the duration of heroin use and age of the subjects, respectively. The results of this study suggest that a comprehensive public health program is needed to prevent transmission of these blood-borne infections. The study's limitations are noted.