Five patients with plasma cell myeloma were studied for skeletal changes on 0.5 to 1 mg sodium fluorid (NaF)/kg/day for 7 to 32 + months. Coarsening of the bony trabeculae was noted by histologic and radiographic studies. The iliac crest of individuals with multiple myeloma had normal ash, fluoride, magnesium, nitrogen, citrate, and hydroxyproline concentrations. The calcium and phosphorus values were somewhat lower with a normal calcium to phosphorus (Ca/ P) ratio. Following fluoride therapy, no changes were seen in ash, calcium, phosphorus, Ca/P ratio, or in nitrogen concentration. Magnesium increased while citrate and hydroxyproline concentrations decreased. Fluoride was incorporated at a rapid rate approaching a steady state some four months after institution of fluoride therapy. The increase in bone crystallinity observed in this study is consistent with the only other crystallographic investigation on the effect of fluoride on bone crystallinity caused by short term treatment of human osteoporotic patients.