This study examined the effect of an HIV-positive notification on the sexual and injection behaviors, living conditions, and medical demands of injection drug users (IDUs) in Montreal. The behavior changes of 73 IDUs who received an HIV-positive test result, and 219 IDUs who tested negative were compared, and the net difference (ND) in the proportion of IDUs adopting the particular change was determined. No significant changes were found in drug use or needle sharing practices. A higher proportion of IDUs who received an HIV-positive notification acquired unstable living conditions, ND = 20.7% (95% CI = 3.3, 38.1), began medical follow up, ND = 34.4% (95% CI = 20.8, 48.7), and increased needle exchange program (NEP) utilization, ND = 20.5% (95% CI = 8.3, 32.8). Compared with HIV-negative males, more HIV-positive male IDUs stopped sexual relations, ND = 24.6% (95% CI = 0.4, 48.9), and sex work, ND = 31.8% (95% CI = 12.4, 51.3), and fewer began new relations, ND = -38.2% (95% CI = -52.6, -23.9). The medical community and NEPs have an important role in providing support for newly diagnosed IDUs.