Of all the environmental factors of our life, drinking water has the greatest effect on our life. Consequently, it is essential to study the relation between drinking water and the caries of temporary teeth, which is of a great importance in the infant hygiene. Nevertheless, we have not seen any report on this study. From this point of view, we carried out medical group examination on the teeth of 1, 835 babies living in the central part of Tokyo and the Izu Seven Islands, who used rain water, underground watr or river water, during the period of 1956-1960. The following result was then obtained by making observation of the caries (def) classified by districts, sources of drinking water, ages, and also generally by the kinds of teeth and the surfaces of teeth.The general observation of def classified by regions revealed that the rate was low for all agegroups in Central Tokyo, while high in the Oshima Island. This tendency was, in addition, remarkable in younger ages. The general observation of def by sources of water revealed the fact that rain water gave the highest rate, while surface water gave the lowest rat, for all ages. Besides, this tendency was most remarkable in two-year-old babies, in whom significant differences were noted as a result of the χ2-test. The observation of two-year-old babies classified by sources of water and kinds of teeth revealed that the teeth related with rain water gave the highest rate, then underground water followed, and river water the lowest, irrespective of the kinds of teeth. Moreover, the tendency was remarkable in the front teeth of the upper jaw and in the molar teeth of the lower jaw. Classified by the surfaces of teeth, it was generally found that the rate was the highest in rain water, then in underground water, and the lowest in river water, for every type of tooth surface. The tendency was remarkable in the occlusion part, incisors and molars on the upper jaw, molar teeth on the lower jaw, and in lower incisors for centripetal and centrifugal lips, cheeks, tongue and palate surface. Compared among the kinds of teeth and surfaces of teeth, def was the highest in the lip surface of the front teeth at the upper jaw, and the tendency was notable when classified by the sources of drinking water.Thus it was discovered that there were characteristic relations among the caries of temporary teeth, region and sources of drinking water, and at the same time it was proved that drinking water played an important role in causing caries in temporary teeth.