Mesenchymal stem cells (MSCs) residing in bone marrow (BM) are the progenitors for osteoblasts and for several other cell types. In humans, the age-related decrease in bone mass could reflect decreased osteoblasts secondary to an age-related loss of osteoprogenitors. To test this hypothesis, BM cells were isolated from vertebral bodies of thoracic and lumbar spine (T1-L5) from 41 donors (16 women and 25 men) of various ages (3- 70 years old) after death from traumatic injury. Primary cultures were grown in alpha modified essential medium with fetal bovine serum for 13 days until adherent cells formed colonies (CFU-Fs). Colonies that stained positive for alkaline phosphatase activity (CFU-F/ALP+) were considered to have osteogenic potential. BM nucleated cells were plated (0.5, 1, 2.5, 5, or 10 x 106 cells/10-cm dish) and grown in dexamethasone (Dex), which promotes osteoblastic differentiation. The optimal plating efficiency using BM- derived cells from donors of various ages was 5 x 106 cells/10-cm dish. BM- derived cells were also grown in the absence of Dex at this plating density. At the optimal plating density, in the presence of Dex, the number of CFU- F/ALP+ present in the BM of the younger donors (3-36 years old) was 66.2 ± 9.6 per 106 cells (mean ± SEM), but only 14.7 ± 2.6 per 106 cells in the older donors (41-70 years old). With longer-term culture (4-5 weeks) of these BM cells in medium containing 10 mM β-glycerophosphate and 100 μg/ml ascorbic acid, the extracellular matrix mineralized, a result consistent with mature osteoblastic function. These results demonstrate that the number of MSCs with osteogenic potential (CFU-F/ALP+) decreases early during aging in humans and may be responsible for the age-related reduction in osteoblast number. Our results are particularly important in that the vertebrae are a site of high turnover osteoporosis and, possibly, the earliest site of bone loss in age-related osteoporosis.