Our study was designed to examine associations of longterm physical impact (running) with changes in lumbar bone mineralization. Study subjects were a volunteer sample of 14 members of a running club now aged 55 to 77 years and 14 matched controls undergoing computerized scans of the first lumbar vertebra both at baseline and after 5 years. Separate analyses included data from 23 runners available over the entire 5-year period. A decrease in bone mineral density (BMD) over time was statistically significant in both runners and controls. Among runners bone loss was most pronounced in those decreasing their running habits substantially. At the 5-year mark, runners maintained greater BMD. A highly significant correlation was found between change in lumbar BMD and average time spent running (min/week) over the 5-year period. Furthermore, changes in BMD were positively correlated with changes in run min/week. We conclude that regular running appears to reduce age related bone loss both in women and men over 50 years of age. However, substantial decreases in physical weight bearing activity are associated with important bone loss in the lumbar spine.