The objectives were (1) to assess caries development, including the incidence and rate of lesion progression, in a Swedish cohort from adolescence to young adulthood and (2) to compare the caries incidence rates in adolescents with those of young adults. The original material consisted of 536 children aged 11-13 years at baseline. This cohort had been followed through annual bitewing radiographs to 21-22 years of age. In 1998-1999, 250 of these individuals were re-examined at the age of 26-27, and the new caries data were added to the original data. The results showed that fewer new enamel lesions developed on approximal surfaces during young adulthood than during adolescence; the caries incidence rates for enamel lesions decreased from 4.3 in the age group 12-15 years to 2.7 new caries lesions/100 surface-years in the age group 20-27 years. The same applied to the rate of lesion progression, where the corresponding values from the enamel-dentin border to the outer dentin were 32.5 for the youngest and 10.9 new lesions/100 surface-years for the oldest age group. The caries incidence of outer dentin lesions on approximal surfaces was low but increased from 0.2 in the age group 12-15 years to 0.9 new outer dentin lesions/100 surface-years in the age group 20-27 years. The incidence rates varied considerably between different tooth surfaces. Also for occlusal surfaces, fewer new dentin lesions developed during young adulthood than during adolescence; the incidence was 2.0 new dentin lesions/100 surface-years in the youngest age group and 0.7 during young adulthood. At the age of 13, the proportion of DFS of occlusal surfaces predominated over DFS of approximal surfaces but at the age of 26-27 the proportions of occlusal and approximal DFS were almost equal.