Physical activity (PA) during physical education is important for health purposes and for developing physical fitness and movement skills. To examine PA levels and how PA was influenced by environmental and instructor-related characteristics, we assessed children's activity during 368 lessons taught by 105 physical education specialists in 42 randomly selected schools in Hong Kong. Trained observers used SOFIT in randomly selected classes, grades 4-6, during three climatic seasons. Results indicated children's PA levels met the U.S. Healthy People 2010 objective of 50% engagement time and were higher than comparable U.S. populations. Multiple regression analyses revealed that temperature, teacher behavior, and two lesson characteristics (subject matter and mode of delivery) were significantly associated with the PA levels. Most of these factors are modifiable, and changes could improve the quantity and intensity of children's PA. (Contains 4 tables.)