A histomorphometric and dynamic study of non-decalcified transiliac biopsies was carried out in 51 cases of osteoporosis who had received double marking with tetracycline before and after two years treatment with association of sodium fluoride (50 mg/day) vitamin, D2 (8,000 IU/day) and calcium (1 g/day). The main effect of fluoride is an increase in the osteoblastic population, which is shown by an increase in the osteoid parameters. The osteoid volume is multiplied by 3,6, the osteoid surfaces by 2,4, and the index of osteoid thickness by 1,2. There exists a lesser increase in the reabsorption surfaces (X 1,2). There results a very significant increase in bony trabecular volume, the average value of which increase from 9,8 +/- 3,1% to 16,6 +/- 9.3% (X 1,8; p < 0.001). These results were found again both in apparently primary osteoporosis and in secondary osteoporosis. No significant depression was noted in the rate of calcification, but six patients developed a state of histological osteomalacia associated in 5 cases with an increased calcified volume. All these results are in good agreement with those of the world literature and indicate that fluoride is able in most cases (60%) to restore normal bones in osteoporosis with reduced risk of fracture.