Endemic waterborne fluorosis is a public health problem in Isparta, a city located in southern Turkey. Fluoride is a cumulative element that increases metabolic turnover of the bone and also affects the homeostasis of bone mineral metabolism. There are number of similarities between the effects of excess parathyroid hormone (PTH) and fluorosis on bone. So fluoride might show its effect via PTH. We aimed to determine PTH levels in patients with endemic fluorosis to estimate the possible toxic effects of chronic fluoride intake. Fifty-six patients with endemic fluorosis and 28 age-, sex-, and body-mass-index-matched healthy controls were included in this study. Endemic fluorosis was diagnosed according to the clinical diagnosis criteria of Wang. The urine fluoride levels of fluorosis patients were significantly higher than those of control subjects as expected (1.9 +/- 0.1 vs. 0.4 +/- 0.1 mg/L, respectively; P < 0.001). PTH levels in fluorosis group were significantly higher than control group (65.09 +/- 32.91 versus 47.40 +/- 20.37, respectively; P = 0.01). The results of our study demonstrate that serum PTH levels are increased in patients with endemic fluorosis. Fluoride, by interfering calcium balance, may be the cause of secondary hyperparathyroidism.