The effect of immobilization on human bone was studied through a longitudinal, as well as cross-sectional, quantitative and dynamic histological analysis of 34 decalcified and undecalcified iliac crest biopsies. They were obtained at various times after the onset of immobilization in 28 patients of which 22 were suffering from post-traumatic spinal cord lesions. Trabecular bone volume, osteoid volume, trabecular osteoclastic resorption surfaces, size of the periosteocytic lacunae, thickness of iliac cortices and volume of the cell population of the marrow were measured. The histodynamic study was made by double tetracycline labeling in 19 patients. The histological data were compared with biological data from another group of 68 immobilized patients including 22 of the patients undergoing biopsy. Calcemia, phosphoremia, alkaline phosphatase, calciura, phosphaturia and hydroxyprolinuria were measured. The decrease of the trabecular bone volume averaged 33% over 25 weeks and then stabilized. Immobilization also caused an early increase in the trabecular osteoclastic resorption surfaces and later in the size of periosteocytic lacunae, an early depression of osteoblastic bone formation and a thinning of the cortices. Calciuria was high, as was hydroxyprolinuria which correlates with resorption surfaces. The histological and biochemical changes suggest an histodynamic hypothesis according to which the global lifespan of the BMU (Basic Multicellular Unit from Frost) would be increased. These changes reflect atransient, leading to a newsteady state:​ rarefied bone with a low rate of subsequent turn-over. They emphasize the importance of mechanical factors in the development of bone cells.