We conducted a histomorphometrical study on the role of the bone marrow in cancellous bone metabolism using iliac bone specimens from 79 postmenopausal women. A gradual decrease in hematopoietic tissue of the bone marrow was proportionate to the decrease in cancellous bone and the ratio of osteoid perimeter/bone perimeter regardless of age. On the other hand, the ratio of eroded perimeter/bone perimeter remained almost steady until hematopoietic tissue decreased significantly. These findings suggest that a change in the bone marrow, that is, a decrease in hematopoietic tissue, causes an imbalance in bone formation and resorption and leads to bone loss.