Disuse osteoporosis induced in vertebrae of the tail distal to an area of resection of the coccygeal nerve and muscles, was studied in four adult cats fed fluoride from the age of six weeks on and in five adult control cats. Bone formation and bone resorption were quantitated by microradiography, and undecalcified bone sections were used to measure osteoid tissue. The development of disuse osteoporosis was not influenced by the presence of fluoride. The increase in bone density in patients with osteoporosis and other bone disease achieved with fluoride therapy may be the result of an increase in bone formation rather than a decrease in bone resorption.