Objectives: To describe the predisposing, enabling and reinforcing factors influencing the levels of physical activity among Canadian youth (ages 12-24). Methods: Analyses of the 1996-97 National Population Health Survey (NPHS) were conducted using data collected from female (n=6195) and male (n=5925) Canadians aged 12-24 years. The data were analyzed using correlational, ANOVA and regression procedures with post hoc analyses (Bonferroni) employed where applicable. Results: Compared to males, Canadian female youth were found to be less physically active, more concerned about being overweight, more depressed, and to consult mental health professionals more frequently. On the other hand, females were more likely to report greater social support than males and to be more socially involved. Those adolescents who smoke tend to be less physically active and, along with those who consume alcohol, have poorer health. Discussion: The significant predisposing, enabling and reinforcing factors influencing youths' physical activity levels more profoundly affect females. The results are discussed in terms of strategies for enabling physical activity available in the literature. In addition to individual determinants influencing physical activity, a move toward understanding policy and environmental factors is recommended for further research.