Crystallographic characteristics of bone mineral were examined in a group of 60 osteoporotic patients before and after 3 to 6 years of fluoride therapy. The age of the mineral was evaluated by means of X-ray absorption, as degree of mineralization of bone tissue (MDBT). Crystallinity was evaluated by measuring both X-ray diffraction line broadening, beta (31.0) and beta (00.2), and the crystallinity index (CI) by infrared spectrometry. The a and c unit-cell parameters were determined by powder X-ray diffraction. Bone fluoride content was measured by specific electrode. Patients were divided in two groups according to MDBT before treatment: one group with MDBT values less than or equal to mean value; another group with MDBT values greater than mean value. In the first group, trabecular bone volume (TBV) did not change significantly during therapy. In the second group, an increase of TBV was observed. Osteoporoses can then be distinguished, on MDBT criterion, between osteoporosis with hypermaturated mineral and osteoporosis with hypomaturated mineral. The MDBT before treatment permits one to predict the effect of fluoride therapy on TBV. In the two groups there was a significant increase in bone fluoride content between the onset and the end of treatment. Bone fluoride content increased linearly during therapy without any plateau effect. Crystallographic modifications induced by fluoride explain the mechanical and chemical improvement of bone.