In order to stady the relation between the mottled tooth, struma and fluorine content of drinking water, the author investigated the teeth and thyroid glands of school-boys and girls living in Yoshioka Hot Spring and its vicinity, Tottori Plefectue. Yoshioka Hot Spring is a simple thermal with high fluorine content (max. 5mg. per liter). The incidence of the mottled tooth was 51.5 per cent in average. A close relationship was observed between the morbidity of the mottled tooth and high fluorine content of drinking water. But the author noticed also the occurrence of the mottled tooth among the school-children living in the regions where the fluorine content of drinking water proved to be even less than o.1ppm., and the author emphasizes the possibility of the causal factors other tham fluoline. The palpability of thyroid gland was 78.4 per cent in average, but no significant relation was recognized between the struma and fluorine. Between the mottled tooth and struma, no relation was proved as significant, too.