In order to find out whether the response of osteoporosis to sodium fluoride treatment or complications arising from this treatment are related to the retention of fluoride in bones, we measured bone fluoride concentrations in 30 patients with primary osteoporosis who had been treated with sodium fluoride for 1 to 8 years. We found a significant correlation (r = 0.74, P less than 0.001) between bone fluoride concentration and total amount of fluoride ingested. However, with any given amount of sodium fluoride, the bone fluoride concentration could vary by up to three times from a patient to another. There was a significant correlation between bone fluoride concentration and trabecular bone volume on the hand (r = 0.54, P less than 0.001) and thickness of osteoid borders measured on the same biopsy specimen on the other hand (r = 0.45, P less than 0.001). Five patients developed lower limb bone fissures after 11 to 18 months of sodium fluoride treatment, but their bone fluoride concentration was not significantly different from that of the 25 patients who had no fracture. These data suggest a relationship between the degree of stimulation of bone formation by fluoride and the bone concentration of that substance; however, peripheral fractures are not dependent on bone fluoride concentration.