In order to find out a parameter for the medical diagnosis and therapeutic criteria of endemic fluorosis, the excretory level of fluorine in urine was determined between the patients of bone fluorosclerosis and those who lived in the district of endemic fluorosis. The results showed that both the patients and the normal people in the endemic area of fluorosis excreted more fluorine in urine than the others, because of drinking highly fluorina-ted water. Thus a positive correlation was found between the amount of fluorine in urine and the degree of fluorinated drinking water, while no significant difference was found between the patients and the normal individuals. The urinary output of fluorine fluctuated in a few cases of fluorosclerosis.From the data obtained we may, perhaps, conclude that the excretion rate of fluorine in urine may be considered as a parameter of fluorine overload in the body, but not as applicable to the medical diagnosis or treatment in fluorosis.