Dental fluorosis was found in nearly 80% of 520 children aged 12 to 18 years who lived permanently in two Greek villages Kolchikon and Lavrion where the water supply contains 3, 6 and 2, 7 ppm of fluoride. Detailed radiological, ophthalmological and biochemical studies in 52 children with marked dental fluorosis revealed that 29 had bone changes in the mandible consisting of erosion or loss of the lamina dura, cystic rarefaction and sclerosis; opacities of the lens were seen in 57% of the patients. Serum Ca was found to be in the normal range in the majority of patients. Serum P,Mg and creatitine as well as blood urea and urinalysis were normal. Serum alkaline phosphatase was generally high in the presence of bone changes. Serum parathyroid hormone was elevated in 10 children (685, 980, 1200, 1280,2450, 2600, 3000, 3100, 6200, 7200 pg/ml) with increased serum alkaline phosphatase and with skeletal changes resembling hyperparathyroidism and normal in four children (185, 280, 420, 630 pg/ml) who had normal serum alkaline phosphatase and no bone changes. It is suggested that fluorosis can cause secondary hyperparathyroidism.