Almost all reports of research on first molars have been studied concerning with dental caries prevalence for a long time. But we thought that the behaviors of sound teeth were more important than these of decayed teeth in public health practice. So we intended to survey the condition of sound teeth of first molars of schoolchildren using the finding of the periodic dental examination in 1973 under direction of school health act. First, the study covered 4,742 children selected from a total sample of about 181,000 individuals from Nagoya, the fourth largest city in Japan. And we investigated as follows: (1) Number of schoolchildren having all sound first molars, (2) Course from sound to decayed teeth of 4 first molars, (3) Prevalence condition of decayed first molars. The results of study were as follows: (1) Percentage of schoolchildren having all sound first molars was 40.2 in male and 29.3 in female at 9 years old. (2) The first decayed tooth of all first molars was found in the lateral side of mandibular and continued to the bilateral side. (3) About 50% of decayed teeth was the first stage of dental caries (enamel caries), but it is very difficult for examiners to make a diagnosis of these teeth. Consequently, it was shown that the criteria of diagnosis of decayed teeth affected the results of health statistics of teeth greatly
