The aim of this study was to evaluate a general school-based 1-year exercise intervention program in a population-based cohort of girls at Tanner stage I. Fifty-three girls aged 7-9 years were included. The school curriculum-based exercise intervention program included 40 minutes/school day. Fifty healthy age-matched girls assigned to the general school curriculum of 60 minutes physical activity/week served as controls. Bone mineral content (BMC, g) and areal bone mineral density (aBMD, g/cm(2)) were measured with dual X-ray absorptiometry (DXA) of the total body (TB), lumbar spine (L2-L4 vertebrae), third lumbar vertebra (L3), femoral neck (FN), and leg. Volumetric bone mineral density (g/cm(3)) and bone width were calculated at L3 and FN. Total lean body mass and total fat mass were estimated from the TB scan. No differences at baseline were found in age, anthropometrics, or bone parameters when the groups were compared. The annual gain in BMC was 4.7 percentage points higher in the lumbar spine and 9.5 percentage points higher in L3 in cases than in controls (both P < 0.001). The annual gain in aBMD was 2.8 percentage points higher in the lumbar spine and 3.1 percentage points higher in L3 in cases than in controls (both P < 0.001). The annual gain in bone width was 2.9 percentage points higher in L3 in cases than in controls (P < 0.001). A general school-based exercise program in girls aged 7-9 years enhances the accrual of BMC and aBMD and increases bone width.