Research on physical activity and fitness indicates their importance in the prevention of disease and promotion of health. However, levels of physical activity have been shown to vary significantly by age, sex and a wide range of additional factors. This paper examines age and sex differences in physical inactivity among 13 to 19 year olds participating in the 1990 Ontario Health Survey. Findings from the bivariate analysis suggest a major increase in physical inactivity between ages 15 and 16. When logistic regression is used to examine the adjusted effects of predictor variables on physical inactivity, the effects of age remain significant. However, the odds of inactivity at one year increments are not significant. There are significant increased odds of inactivity associated with such factors as sex, friends' (activity) participation, perceived future health problems, and perceived health status. The public health implications of the findings suggest that those responsible for developing programs and policies to increase physical activity among teenagers should consider the critical years of decreased activity and the factors that might explain why this decline occurs.