Objectives: To characterize trends in drug and sexual risk factors among injection drug users (IDU) according to their age at first injection.Methods: Data were collected through the Vancouver Injection Drug Users Study (VIDUS). To date over 1400 Vancouver IDU have been enrolled and followed. Semi-annually, participants have completed an interviewer-administered questionnaire and have undergone serologic testing for HIV and Hepatitis C (HCV). These analyses are based on baseline data and supplementary questions that were added during two consecutive follow-up questionnaire periods in 2001 concerning initiation into injection drug use. Cochran-Armitage Trend tests were used to detect trends between three categories; persons who began using injection drugs at age £15, 16 - 18, and ³19. Results: There were 1452 participants with baseline data. There were clear associations between younger age at first injection and higher proportions of females (p=<0.001), childhood sexual abuse (p=<0.001), incarceration (p=<0.001), sex trade work (p=0.004), daily heroin (p=<0.001), cocaine (p=0.001), and polydrug (p=<0.001) use. In the sub-analyses in which specific initiation questions were asked there were 893 participants with available data. Of those, there were clear trends across the three age categories for; contact with the foster care system (p=<0.001), injecting with a needle used by someone else (p=0.046), having parents or caregivers who used injection drugs (p=<0.001), having siblings who also inject (p=0.001), and recent sexual abuse (p=0.001).Interpretation: The despair caused by sexual abuse and being raised in the foster care system, particularly among young females, may provide insight into why youth begin to inject drugs. These findings may be useful in terms of targeting prevention programs and also raising awareness and empathy surrounding the use of injection drugs. [abstract]