145 women showing clinical and radiological signs of involutional osteoporosis of the spine were biopsed at the ilium for histomorphometric analysis of bone mass including trabecular bone volume and parameters reflecting the spatial distribution of bony elements (mean trabecular plate thickness, density and separation). Results were compared with an age-matched population of 22 healthy females. Postmenopausal osteoporotics (i.e. younger than 75 years) were characterized by a significant reduction in trabecular bone volume, plate density and thickness, while senile osteoporotics (i.e. older than 75 years) did not exhibit any difference with controls. 51% of the osteoporotic patients had a trabecular bone volume higher than the spontaneous vertebral crush threshold defined by Meunier. Osteoporotic patients with trabecular bone volume under the vertebral crush threshold had a significant decrease in all trabecular parameters. On the opposite, patients with trabecular bone volume above the vertebral crush threshold had only a significant decrease in the number of trabeculae. A negative correlation was found between age and plate density in both osteoporotic patients and controls. Alinear correlation was found between trabecular bone volume and plate density, but thickness and density of trabecular plates were not correlated. This study confirms that involutional osteoporosis is not only a decreased bone mass disorder. A modified spatial distribution of trabeculae or a«mechanically less resistant bone matrix could be additional factors