In 14 patients being treated with fluoride preparations for osteoporosis histomorphometric analysis of iliac biopsies showed that the volume of the mineralised spongiosa had increased by more than 20% above initial level in eight instances. The volume of the osteoid in ten cases had also increased, generally most markedly in the first year. In some cases the clinical symptoms corresponded to the histomorphometric findings. The results suggest that fluoride treatment of osteoporosis can be continued beyond one year. But this should only be done under precise control also of histological bone studies because in three of the patients the spongiosa volume had reached high normal values already after one to three years of treatment.