This study investigated the occurrence of caries among 15-year-olds using the municipal oral health service in Helsinki in 1976 and in 1986. Data were collected from each subject's personal dental health file as recorded during routine check-ups by dentists in the health centre of the city of Helsinki in the year in question. All indicators showed a significant improvement (p less than 0.001) in dental health during the 10-year period. The percentage of intact teeth and surfaces increased markedly. The mean number of DMF teeth per person fell from 12.1 to 5.1 and that of proximal DF surfaces from 5.6 to 1.5. In the 1976 group, 63% of all occlusal surfaces had been filled or were decayed. Ten years later the figure was 26%. Proximal DFS percentages were 10 and 3%, respectively. The ratios of the total number of proximal to occlusal DFS were 0.6 in 1976 and 0.4 in 1986.