An epidemiological study of the prevalence of dental caries and gingivitis in 13-year-olds was carried out in the county of Västerbotten, Sweden, in 1987. Besides providing information on the dental health of this age group, prevalence data were compared with data from earlier surveys of the same age group carried out in 1967 and 1977, covering the same areas and applying the same diagnostic criteria and methods. The results showed a marked decrease in the prevalence of caries and gingivitis. The mean number of DMF surfaces was 4.6, compared to 9.0 in 1977 and 19.0 in 1967. The mean GBI% was reduced from 33% to 21% between 1977 and 1987. In contrast to the results from 1967 and 1977 the differences in mean DMF values between the three areas were nonsignificant in 1987. The decline in dental caries is most probably due to an extended regular dental care with a strong preventive approach. Only slight differences in dental health were found between the three areas in 1987 in contrast to in 1977 and 1967. The most likely reason was probably that larger resources had been allocated to preventive programmes in the rural areas than in the city of Umeå.