Physical activity engagement during physical education is important for many reasons, including developing physical fitness and movement skills and promoting health. Much more is known about physical activity in elementary than secondary schools. We examined physical activity and how it was influenced by instructor-related and environmental characteristics during 238 lessons taught by 65 physical education specialists in 30 randomly selected secondary schools in Hong Kong. Trained observers used SOFIT (System for Observing Fitness Instruction Time) in randomly selected grade 7-12 classes over a 6-month period. Results showed students engaged in moderate-to-vigorous physical activity (MVPA) about 35% of lesson time, a level similar to that found in U.S. elementary schools and short of the U.S. Healthy People 2010 objective of 50% engagement time. Multiple regression analyses found that six potentially modifiable variables contributed to 35% of the variability in lesson MVPA percent. (Contains 4 tables.)