The purpose of this study was to assess physical activity levels during high school physical education lessons. The data were considered in relation to recommended levels of physical activity to ascertain whether or not physical education can be effective in helping young people meet health-related goals. Sixty-two boys and 60 girls (aged 11-14 years) wore heart rate telemeters during physical education lessons. Percentages of lesson time spent in moderate-and-vigorous (MVPA) and vigorous intensity physical activity (VPA) were recorded for each student. Students engaged in MVPA and VPA for 34.3 +/- 21.8 and 8.3 +/- 11.1% of lesson time, respectively. This equated to 17.5 +/- 12.9 (MVPA) and 3.9 +/- 5.3 (VPA) min. Boys participated in MVPA for 39.4 +/- 19.1% of lesson time compared to the girls (29.1 +/- 23.4%; P < 0.01). High-ability students were more active than the average- and low-ability students. Students participated in most MVPA during team games (43.2 +/- 19.5%; P < 0.01), while the least MVPA was observed during movement activities (22.2 +/- 20.0%). Physical education may make a more significant contribution to young people's regular physical activity participation if lessons are planned and delivered with MVPA goals in mind.