In consideration of dental caries of deciduous teeth, it is important to examine the fact that formation of toothgerm and calcification of deciduous teeth are made in the intrauterine; and also native and acquired characters on the dental caries should be examined because of the eruption on the deciduous teeth after birth.The present study, therefore, was performed on the elements of climate which are the outside-environment of pregnant women and dental caries of deciduous teeth, that is, on the relationship among temperature, humidity, wind velocity and discomfort index.As the subjects of the study, 4, 992 children in Tokyo and their 93, 148 teeth were employed.The subjects were classified into the group of age at first, and then, this group was divided into groups of the month of birth.And comparison was made on the percentage of def experienced children, the average number of def teeth per child, the percentage of def experienced teeth, as well as the percentage of def experienced teeth classified by the kinks of teeth, the month of birth of children classified by the month of birth, and also on the monthly average values of the temperature, humidity, wind velocity, and discomfort index from the one to the five months before birth of children. The calculation was performed on the minimum and maximum zones of def from the relationship between def and elements of climate.(1) In the def of subjected children, the percentage of def experienced children was 63.16% the average number of def teeth per child was 4.34 and percentage of def experienced teeth was 22.64 %.(2) As for def, its highest value was observed generally in the children born in September and October in either group of age, but low values were observed in those children born in May, June and July. This tendency was most remarkable in lower age-groups.(3) The similar tendency was observed in def classified by the kinks of teeth. Especially, it was remarkable on the teeth with high def values.(4) As for the relationship among def of children, elements of climate and discomfort index in fetal life, the def of children classified by the month of birth in the group of age 5wes related to the temperature and discomfort index in one month before birth.The def of children classified by the month of birth in the groups of age 4 and 5 was related to the humidity in one month before birth, and also the def of children classified by the month of birth in the group of age 2 was related to the wind velocity in one and two months before birth.(5) In the def classified by the kinds of teeth, the def of the upper primary central and lateral incisors in the group of age 5 was related to the temperature, humidity and discomfort index in the month of birth.The def of the upper and lower first primary molars in the group of age 5 was related to the temperature, humidity and discomfort index in two months before birth, and also the def of other teeth in the group of age 4 and 5 was related to that of one month before birth.(6) Minimum and maximum zones of dental caries of deciduous teeth in all round-elements of climate; regarding the children of month in age groups related to def classified by the kinds of teeth and elements of climate, the minimum and maximum zones of def and the rate of appearance on maximum and minimum values were calculated in the use of the values of temperature, humidity and discomfort index on the minimum and maximum values of def.As the result, in the minimum zone of def, the lowest was 14.5°C in the temperature, 62 % in the humidity: the highest was 12.6°C in the temperature, 73 % in the humidity; and the following equations were obtained:log y=0.0001x+0.99y=0.2651x2-35.958x+1224(y: temperature, x: humidity)