The purpose of this study was to analyse the establishment of the toothbrushing frequency as a healthy habit by Finnish adolescents between 12 and 18 years of age. The data were collected as part of the nation-wide research programme, the Adolescent Health and Lifestyle Survey. All Finns born in 1968 with birthdays on 20-25 July formed the sample (N=1106). Questionnaires were mailed to the whole sample in February 1981 (12 years of age), -83 (14 years), -85 (16 years) and -87 (18 years). The rate of return for all four questionnaires was 62 percent (six-year follow-up) and for two questionnaires (two-year follow-up studies) 79 percent (12 and 14 years), 72 percent (14 and 16 years) and 68 percent (16 and 18 years). Those adolescents who brushed their teeth more than once a day at age 12, were more stable in their behaviour during the following six years than those who brushed their teeth less often. Over two thirds of the respondents, both boys and girls, indicated one or more changes in their toothbrushing frequency in the four questionnaires during the six-year follow-up. The rate of improvement in toothbrushing frequency was higher among girls than boys, and the rate of deterioration in toothbrushing frequency was higher among boys than girls. One sixth of the adolescents still showed a deterioration in their brushing frequency between the ages of 16 and 18. It may be concluded that although toothbrushing frequency will be gradually adopted as a health habit when the adolescents mature between 12 and 18 years, only half the girls and one fifth of the boys adopt the recommended frequency. Health education should still improve performance in those groups which have not established the recommended toothbrushing frequency.