The elimination of gonorrhoea and a significant reduction in chlamydia are proposed as national goals that Canada should achieve by the year 2010 (Health Canada, 1997). Already the national incidence of gonorrhoea has declined by 27% and chlamydia by 66% since 1991 (Health Canada, 1998a). However, the overall rates are influenced by very high rates in certain vulnerable segments of the population, such as youth living away from their parents/guardians (Health Canada, 1998b). Because of their high STD rates, these street-involved youth are a priority for STD prevention programmes. Understanding more about the determinants of STD risk within this group is a necessary step in reducing sexual risk-taking activities and, consequently, STD rates. In an attempt to provide information on the sexual health and sexual behaviours of Canadian street youth, the Laboratory Centre for Disease Control (LCDC, Health Canada; now the Centre for Infectious Disease Prevention and Control (CIDPC; circa 2001)) developed the Enhanced STD Surveillance in Canadian Street Youth project. This national, multi-centre, cross-cultural surveillance system represents the first time that a Canadian national body has examined the prevalence of STD and the determinants of sexual risk behaviours among Canadian street-involved youth, The current study used structural equation modelling to test the explanatory power of a tri-partite model incorporating the constructs of personal attributes, behavioural repertoire, and interpersonal environment on STD risk. It was found that a model including self-esteem, perception of risk, abuse history, relationship with parents, current alcohol use, and current illicit drug use explained a significant portion of the variance in STD risk, as defined by (a) current frequency of condom use, (b) number of recent sexual partners, (c) past and current incidence of chlamydia, gonorrhoea, and/or hepatitis B, and (d) use of used injection drug use equipment. These results suggest that STD risk among street-involved youth is a complex social problem that requires a comprehensive prevention framework regarding determinants of health.