Combined treatment with sodium fluoride, calcium and vitamin D is claimed to be beneficial in some patients with osteoporosis. Fluoride alone leads to accumulation of unmineralised bone, producing the histological picture of osteomalacia. The addition of calcium or vitamin D, or both, is believed to prevent this complication. We report a case where osteomalacia developed during sodium fluoride treatment despite large doses of vitamin D and associated high plasma 25-hydroxyvitamin D (25-OHD) concentrations.