The purpose of this histomorphometric study of iliac bone biopsies from 10 postmenopausal osteoporotic patients was to describe the effects of sodium fluoride (combined with calcium and vitamin D) on remodeling in cortical bone after 6 months and after 5 years of treatment. Biopsies had been fixed in absolute methanol, embedded undecalcified in methylmetacrylate, and cut on a heavyduty microtome. The therapy had no effect on the thickness of cortical bone in the iliac crest but increased the porosity slightly. It had no statistically significant effect on depth of resorption or thickness of new walls formed at remodeling sites but treatment increased the fraction of osteons undergoing remodeling in cortical bone. After 6 months of treatment, the increase was due to an enhanced activation of new remodeling sites, but in biopsies taken after 5 years of treatment, some degree of mineralization defect was observed and the duration of the remodeling cycle appeared to be prolonged. The mechanism underlying this qualitative change in the response to treatment is unknown, and it is unclear whether the mineralization defect may be prevented by, e.g., an altered supplementation of vitamin D or calcium.