The densities and cross-sectional areas (CSAs) of the femur in 40- and 60-week-old senescence-accelerated mice prone 6 (SAMP6) were determined using peripheral quantitative computed tomography (pQCT) and compared with those in age-matched control mice (ICR). In addition, the effects of 20 weeks of voluntary running exercise with increasing loads on femur density and CSA were examined in SAMP6. There were no differences in the cortical and trabecular densities or CSAs between the 40- and 60-week-old ICR. The cortical and trabecular densities of the 60-week-old control SAMP6 were lower than those of the 40-week-old control SAMP6. There was no difference in the cortical CSA between the 40- and 60-week-old control SAMP6, while the trabecular CSA of the 60-week-old control SAMP6 was higher than that of the 40-week-old control SAMP6. It was concluded that an age-related decline in femur cortical and trabecular densities occurs at an earlier age in control SAMP6, indicating that SAMP6 show accelerated progression of age-associated osteoporosis. There were no differences in the cortical density between the 40-week-old control and 60-week-old exercised SAMP6. These data indicate that running exercise with increasing loads prevents age-associated osteoporosis in this senescence-accelerated mouse strain.