Nine patients with skeletal fluorosis were subjected to iliac crest biopsy because they presented with stiffness and bone pains. The histopathological findings are correlated with the clinical course, X-ray and laboratory data. All but one of the patients showed an increase in bone surfaces lined by osteoid and in these eight patients resorption of bone was the most striking finding and in one case the bone marrow was predominantly fibrous. The severity of radiological and histological bone changes appeared to increase with the duration of exposure to fluoride water in the majority of the cases. The increase in resorption surfaces correlated, positively, with plasma levels of alkaline phosphatase, bone fluoride and the radiological findings of coarse striated patterns and erosions in the bone. The histopathological picture had no relationship with the plasma calcium and the urinary excretions of calcium and fluoride. The concentration of fluoride in the iliac crest biopsies was related to the degree of bone resorption in majority of the cases and the highest level in these patients was in the one who had stayed longer in the endemic fluorosis area. New bone formation and osteoclastic resorption were probably caused by the increased action of parathyroid hormone secondary to the fluorosis. The hyperfunction of the parathyroid glands developed as a compensatory phenomenon to maintain the plasma calcium in the presence of the fluoroapatite crystals which are known to be less soluble and less reactive.