A randomized double-blind study was carried out in 26 patients with multiple myeloma to compare the therapeutic effect of sodium fluoride (50 mg twice daily) plus calcium carbonate (1 g four times daily) and placebo. All patients also received melphalan and prednisone for one week every six weeks. Bone biopsies for microradiography and histology, and videodensitometry as well as conventional roentgenograms, 99mTc-polyphosphate bone scans, and bone densitometry of the mid and distal radius, were done initially and one year after therapy. Microradiography and videodensitometry studies revealed significant increases in bone formation (P less than 0.01) and bone mass (P less than 0.005) in the fluoride-calcium group. Bone trabeculae appeared thickened on roentgenograms of six of 13 fluoride-calcium-treated patients (P less than 0.02). Technetium bone scans and bone densitometry determinations proved insensitive for detection of skeletal changes. Fluoride calcium should be considered a useful adjunct in the treatment for multiple myeloma.