Data for this study were collected from four biennial cycles of OSDUS conducted during 1999-2005. OSDUS is a cross-sectional survey conducted by the Centre for Addiction and Mental Health since 1977 to assess the prevalence of health risk behaviors among youths in Ontario, Canada.3 In each of the four OSDUS surveys, respondents were selected using a two-stage cluster sample with a probability design that permitted representative sampling of all students in grades 9-12 who attended publicly funded schools in Ontario. The two stages of sample selection consisted of schools and classes, both of which were stratified by region and type of school. The total sample for the study described in this report consisted of 13,260 students in grades 9-12 who completed self-administered, anonymous questionnaires in the classroom during a regular class period under the supervision of trained data collectors every 2 years during 1999-2005. In 1999, 2001, 2003, and 2005, sample sizes were 1,495, 1,278, 4,693, and 5,794, respectively; student completion rates were 76%, 71%, 72%, and 72%, respectively; and school participation rates were 90%, 74%, 88%, and 95%, respectively. During 1999-2005, response rates by grade ranged from 70% to 77% for grade 9, from 68% to 76% for grade 10, from 68% to 73% for grade 11, and from 68% to 76% for grade 12. The survey questions, which were adapted from the Youth Risk Behavior Survey (YRBS) cited in U.S. reports,2,4 were as follows: (1) “Are you in enrolled in a PE class?” (defined as attending a PE class on 1 or more days in an average week when in school), (2) “Do you attend PE daily?” (defined as attending PE class for 5 days in an average week when in school), and (3) “On how many of the last 5 school days did you participate in physical activity for at least 20 minutes that made you sweat and breathe hard in physical education class in your school?” (defined as reporting ≥20 minutes of vigorous physical activity during an average PE class 3-5 days per week). All four OSDUS surveys were approved by the Research Ethics Board of the Centre for Addiction and Mental Health.