The purpose of this study was to examine trends in caries prevalence during 1970-90 in the junior high school students aged 12-15. A total of 840 children from 1st (12-13-yr-olds) to 3rd (14-15-yr-olds) school grade in the junior high school in Shizuoka city, Japan, was examined in 1990. The caries prevalence in 1990 was then compared with the data which had been collected from the longitudinal survey by Katayama (1970-72) and the cross-sectional survey (1985) on schoolchildren in the same junior high school. The results indicated that DMFT and DMFS indices in the 1st school grade in 1990 were significantly lower than in 1985 (p<0.05) and in 1970-72 (p<0.01). The DMFT indices at age 12-13 were 5.47 in 1970-72, 3.60 in 1985, and 3.09 in 1990. Percentage decreases in DMFT at age 12-13 were observed in the maxillary incisors (36%), maxillary molars (21%), and lower molars (13%) during 1985-90. The percentage decrease per annum of DMFT index at the age of 12-13 in the examined population was relatively low (-2.9%) in comparison with other industrial countries. Likewise, the downward trends in caries prevalence were apparent in the 2nd and 3rd school grades during 1971-85, but no significant change was observed between 1985 and 1990. The reason for the change during 1985-90 in this population is not clear, but the study shows that the frequency distributions provide valuable information for monitoring changes and high risk groups in caries prevalence.
