For more than 30 years, sodium fluoride has been a commonly used therapeutic agent for established osteoporosis because of its repeatedly documented anabolic effect on trabecular bone mass. Recent clinical and experimental studies have, however, indicated a possible detrimental effect of fluoride on bone strength. Thus, the efficacy of fluoride therapy remains a controversial issue. The aim of this study was to investigate the effect of fluoride on both vertebral bone mass and quality in rats. Twenty-nine 3-month-old, female rats were randomized into three groups. One group served as a control group, and the other two groups received fluoridated water at different doses (100 ppm and 150 ppm). The rats were followed for 90 days. Three lumbar vertebrae were obtained from each rat, and changes in bone fluoride content, bone mass and biomechanical competence were assessed. The results revealed a significant increase in bone fluoride content, ash density and trabecular bone volume after fluoride treatment. Directly obtained load values and load corrected for cross-sectional area were constant. Load corrected for ash content, which is a measure of bone quality, decreased significantly after fluoride therapy. It is concluded that the increase in bone mass during fluoride treatment does not translate into an improved bone strength and that the bone quality declines. This investigation thereby supports the hypothesis of a possible negative effect of fluoride on bone quality.