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People who inject illegal drugs are known to be atheightened risk of HIV infection and other blood-borne diseases. In 2007, more than 20 % of all new cases of HIV infection recorded in Canada were attributed to injection drug use.1 Behaviours associated with the use of particular injection drugs (e.g., cocaine and heroin) have been identified as key factors driving HIV transmission among drug users in various settings.2 However, over the last decade, significant changes in the popularity of specific illegal drugs have been observed.3,4 In recent years, cities across Canada have experienced an explosive increase in the use of crack cocaine, whereas some drugs, including heroin, appear to be less commonly used.5–7 The proportion of drug users who smoke crack cocaine and are homeless or have marginal housing has recently been reported to be as high as 86.6 % in Vancouver, 66.7 % in Edmonton and 62.4 % in Toronto.8 In the United States, a seminal cross-sectional study involving inner-city young adults showed that smoking of crack cocaine was associated with HIV infection.9 However, in Canada, despite the documentation of changing patterns of drug use in many communities,8 little is known about the impact that increased smoking of crack cocaine has had on the HIV epidemic. This is problematic because public health programs for people who smoke crack cocaine have been highly controversial in Canada.6 We conducted a longitudinal study to evaluate whether smoking of crack cocaine has emerged as a risk factor for HIV infection among people who inject drugs. Methods Study design We obtained data from the Vancouver Injection Drug Users Study, an open prospective cohort study that has been ongo-ing since May 1996. Details of that study’s methods have been described previously.10 In brief, eligibility is limited to people who, at recruitment, report injecting illicit drugs a