Daily treatment with 30 mg of sodium fluoride (NaF) and 1 g of calcium over a 3-year period increased the bone mineral content (BMC) in the spines of women (n = 25) with osteoporosis. Determination of the BMC was followed with dual photon absorptiometry (137Cs-241Am) in the third lumbar vertebra. No increase in BMC was found with only 10 mg sodium fluoride in combination with calcium (n = 25), with calcium alone (n = 25), or with placebo (n = 25). No serious side effects were registered. There was, however, minor gastrointestinal distress in one-fifth of the patients taking 30 mg NaF daily.