The study aim was to evaluate the effectiveness of a 2-year health-related physical education intervention in a pretest-posttest design. Sixteen elementary schools (764 pupils, mean age: 11.2 plus or minus 0.7) participated in the study. Schools were randomly assigned to the intervention condition (n = 8) and the control condition (n = 8). Making use of direct observation data gathered according to SOFIT (System for Observing Fitness Instruction Time), the moderate-to-vigorous physical activity engagement during physical education classes was significantly higher in the intervention condition than in the control condition. Children's moderate-to-vigorous physical activity engagement during physical education lessons increased with 14% in the intervention condition (from 42 to 56%). No significant effects were found on the accelerometer data. The health-related physical education intervention was found to be promising in promoting physical activity during physical education classes. (Contains 3 tables.)