OBJECTIVES: Few prospective studies are available on the relationship between incarceration and HIV risk among injection drug users (IDUs). The authors evaluated self-reported rates of syringe sharing and incarceration among a cohort of IDUs. METHODS: This study analyzed syringe lending by HIV-infected IDUs and syringe borrowing by HIV-negative IDUs among participants enrolled in the Vancouver Injection Drug Users Study (VIDUS). Since serial measures for each individual were available, variables potentially associated with each outcome (syringe lending and borrowing) were evaluated using generalized estimating equations for binary outcomes. RESULTS: The study sample consisted of 1,475 IDUs who were enrolled into the VIDUS cohort from May 1996 through May 2002. At baseline, 1,123 (76%) reported a history of incarceration since they first began injecting drugs. Of these individuals, 351 (31%) reported at baseline that they had injected drugs while incarcerated. Among 318 baseline HIV-infected IDUs, having been incarcerated in the six months prior to each interview remained independently associated with syringe lending during the same period (adjusted odds ratio [OR]=1.33; 95% confidence interval [CI] 1.06, 1.69; p=0.015). Similarly, among the 1,157 baseline HIV-negative IDUs, having been incarcerated in the six months prior to each interview remained independently associated with reporting syringe borrowing during the same period (adjusted OR=1.26; 95% CI 1.12, 1.44; p<0.001). CONCLUSIONS: Incarceration was independently associated with risky needle sharing for HIV-infected and HIV-negative IDUs. This evidence of HIV risk behavior should reinforce public health concerns about the high rates of incarceration among IDUs.