['Most prospective pediatric exercise intervention studies cover <1 year and use bone traits as surrogate end points for fractures. This prospective controlled exercise intervention study therefore followed not only skeletal development but also fracture incidence for 4 years.', 'Fractures were prospectively registered in a cohort of children aged 7 to 9 years, 446 boys and 362 girls in the intervention group (2675 person-years) and 807 boys and 780 girls in the control group (5661 person-years). The intervention included 40 minutes per day of school physical education for 4 years whereas the controls had 60 minutes per week. In a subsample, 73 boys and 48 girls in the intervention and 52 boys and 48 girls in the control group, bone mineral content (g) and bone width (cm) were followed by means of dual-energy radiograph absorptiometry.', 'The rate ratio for fractures was 1.11. In the dual-energy radiograph absorptiometry-measured children, there were no group differences at baseline in age, anthropometrics, or bone traits. The mean annual gain in lumbar spine bone mineral content was 7.0% higher in girls and 3.3% higher in boys and in femoral neck width 1.7% higher in girls and 0.6% higher in boys in the intervention than in the control group.'] Does a 4-year exercise program in children increase bone mass without increasing fracture risk?