A 20-30 years' occupational and/or environmental exposure to fluorine may cause osteosclerosis, especially of the spine and pelvic bones, and calcification of spinal ligaments. The radiological picture may mimic many other diseases, especially ankylosing spondylitis, diffuse idiopathic hyperostosis, and rare bone diseases, such as Albers-Schönberg disease. As clinical findings are often not characteristic, early diagnosis is based on the history of occupational exposure, examination of urinary fluoride excretion (over 8 mg F-/litre in 24 hours) and radiological signs. It is also useful to monitor the concentration of atmospheric fluoride.