An osteosclerosis opacifying the axial skeleton and affecting in particular all of the spine, was observed in two women aged 75 and 42 years who had been suffering from a rheumatoid arthritis developing between 15 and 26 years. It was traced to a chronic fluorine intoxication, stemming from the regular taking, for 10 years and 8 1/2 years, of a non cortisone, anti-inflammatory niflumic acid. This fluorine product has 3 atoms of fluor in its molecule (50.0 mg for a tablet of 250 mg). Its administration to control subjects proved the production of ionized fluor by way of the metabolism, and the accumulation of fluor in the organism. Rheumatoid polyarthritis and the prolonged corticotherapy (10 mg of prednisone per day for 21 years) cannot be dismissed as the origin of the severe demineralization of the limbs observed in the second patient, but the role of fluorine seems marked in the occurrence of this peripheral involvement with problems of mineralization and secondary hyperparathyroidisms. On the other hand, the absence of an intervertebral narrowing in the 2 patients, despite the very prolonged taking of cortisone (5 mg of prednisone per day for 15 years, for the 75-year-old patient) is perhaps a result of the fluorine.