AbstractThe aim of the study was to investigate the usefulness of urinary fluoride excretion in evaluating fluoride therapy. In a prospective study, 35 patients with osteoporosis were treated for about 44 months with a mean dosage of 31.4 mg fluoride ion per day. Urinary fluoride excretion and serum alkaline phosphatase activity were measured at 3‐month intervals. Bone mineral content (BMC) was measured in L2–L4 with dual‐photon absorptiometer. The mean number of BMC measurements was 5.7 per patient. The interindividual reproducibility for measurements in 10 patients was 2.1%. For each individual, the regression coefficient of BMC for the period of treatment was calculated. Responders were defined as those who had a positive value and nonresponders had 0 or a negative value. The percentage responders was 83%. Between responders and nonresponders no differences were found for age, fluoride dosage, duration of treatment, or changes in serum alkaline phosphatase activity. Urinary fluoride excretion was higher in responders than in nonresponders (p<0.001) and a positive correlation (p8 mg/24 h. All nonresponders had a urinary fluoride excretion<8 mg/24 h. Urinary fluoride excretion is a valuable predictor of BMC response during fluoride therapy for osteopo