Not much is known about the osteogenic effects of sport activities before puberty. We tested the hypothesis that football (soccer) participation is associated with enhanced bone mineral content (BMC) and areal density (BMD) in prepubertal boys. One hundred four healthy white boys (9.3 +/- 0.2 years, Tanner stages I-II) participated in this study: 53 footballers and 51 controls. The footballers devoted at least 3 h per week to participation in football, while the controls did not perform in any kind of regular physical activity other than that programmed during the compulsory physical education courses. Bone variables were measured by dual-energy X-ray absorptiometry. The maximal leg extension isometric force in the squat position with knees bent at 90 degrees and the peak force, mean power, and height jumped during vertical jumps were assed with a force plate. Additionally, 30-m running speed, 300-m run (anaerobic capacity), and 20-m shuttle-run tests (maximal aerobic power) were also performed. Compared to the controls, the footballers attained better results in the physical fitness test and had lower body mass (-10%, P < 0.05) due to a reduced percentage of body fat (4% less, P < 0.05). The footballers exhibit enhanced trochanteric BMC (+17%, P < 0.001). Likewise, femoral and lumbar spine BMD were also greater in the football players (P </= 0.05). The femoral and lumbar BMC, and to a lower extent the BMD, were correlated with the lower limb muscle mass and the whole body lean mass. Interestingly, among all physical fitness variables, the maximal isometric force showed the highest correlation with total and regional BMC and BMD. Multiple regression analysis indicated that the 30-m running speed test, combined with the height and body mass, has predictive value for whole-body BMC (r = 0.92, P < 0.001) and BMD (r = 0.69, P < 0.001) in prepubescent boys. In summary, football participation is associated with improved physical fitness, reduced fat mass, increased lean body and BMC masses, and enhanced femoral and lumbar spine BMD in prepubertal boys. The combination of anthropometric and fitness variables may be useful to detect children with potentially reduced bone mass.