The authors characterized marijuana smoking among young adult Canadians, examined the co-morbidity of tobacco and marijuana use, and identified correlates associated with different marijuana use consumption patterns. Data were collected from 20,275 individuals as part of the 2004 Canadian Tobacco Use Monitoring Survey. Logistic regression models were conducted to examine characteristics associated with marijuana use behaviors among young adults (aged 15-24). Rates of marijuana use were highest among current smokers and lowest among never smokers. Marijuana use was more prevalent among males, young adults living in rural areas, and increased with age. Young adults who were still in school were more likely to have tried marijuana, although among those who had tried, young adults outside of school were more like to be heavy users. Males and those who first tried marijuana at an earlier age also reported more frequent marijuana use. These findings illustrate remarkably high rates of marijuana use and high co-morbidity of tobacco use among young adult Canadians. These findings suggest that future research should consider whether the increasing popularity of marijuana use among young adults represents a threat to the continuing decline in tobacco use among this population.