According to a cohort study conducted in Montréal, Québec from 1995 to 2000, the mortality rate among street youth was 921/100,000 person-years. Several new community initiatives aiming to increase access to housing and to social and health services for the homeless were implemented in the city between 2000 and 2003. This study aims to update the mortality rate estimate for the period 2001-2006 and to examine factors that could explain a difference between rates, if any. A second cohort study was conducted between 2001 and 2006. The Cohort 2 mortality rate was computed and compared with the Cohort 1 rate. Several analyses were then carried out: (1) mortality rates in the general population were compared with street youth rates using standardized mortality ratios (SMR); (2) Cohorts 1 and 2 distributions of risk factors for mortality were examined, and their effects were assessed using multivariate proportional hazards regression analyses carried out on a combined Cohorts 1 and 2 dataset. Mortality rate among street youth decreased by 79% while it declined by only 19% in the general population; the SMR for Cohort 1 was higher than for Cohort 2 (11.6 versus 3.0). Multivariate proportional hazards regression analyses yielded estimates that were close to the model's estimates based on Cohort 1 data only, and participation in Cohort 1 was an independent predictor of mortality, with an adjusted hazard ratio of 9.0. The mortality decline cannot be completely explained by a similar decrease among the general population or by a difference in distribution of risk factors for mortality between the two cohorts. Field workers suggested that the decrease in heroin consumption they had observed in the streets might have contributed to the mortality decline. We then performed additional analyses which showed that even though the proportion of street youth currently using heroin decreased significantly between 1995 and 2005, the association between heroin use and mortality was not significant (adjusted hazard ratio of 0.9; 95% CI, 0.4 to 2.3). It seems that various factors could have contributed to the decline in mortality rates among street youth, one plausible factor being the implementation of new services for the homeless. This study underscores the importance of monitoring risky behaviors among vulnerable populations to ensure that morbidity and mortality data among these populations is correctly interpreted. Setting up a system to monitor the drug market could improve the quality of information collected.