National epidemiologic data on oral health in children and adolescents in Sweden are restricted to caries, such as the number of decayed and filled teeth (DFT) and decayed and filled surfaces (DFS). Information about more complicated and time-consuming procedures such as endodontic treatment is scarce. The aims were to study the prevalence, quality and potential risk factors for endodontic treatment in young permanent teeth. The material consisted of dental records and radiographs. The subjects were all 19-year-olds born in 1979 (paper I-III) and all 15-year-olds born in 1990 (paper IV) belonging to the public dental clinics in Malmö. Paper III also included a control group. Aims and results of the four papers in this thesis: Paper I. The prevalence and causes of endodontic treatment were studied. Nine percent of the individuals had at least one endodontically treated tooth; in the majority of the cases the cause was caries. The most commonly treated tooth was the first molar. Paper II. The periapical status and technical quality of these rootfilled teeth were studied. Apical periodontitis and inadequately sealed teeth were found in about half of the cases. Technical quality was statistically significantly correlated with periapical status at follow-up. Paper III. The association between potential risk factors and future endodontic treatment due to caries was assessed. Individuals with endodontic treatment had more caries at age 10, more often dental anxiety and more missed dental appointments than a control group with no endodontic treatment. Paper IV. The prevalence of unrestored dentin caries, frequency of deep restorations, and lesion progression between ages 14 and 15 was evaluated. Twenty-two percent of the individuals had one or more surfaces with dentin caries left unrestored. About one out of five had one or more surfaces with deep dentin restorations. Nine percent of the dentin lesions progressed to deep dentin lesions. Conclusions: Endodontic treatment was fairly common in 19-year-olds and caries was the most common cause. The technical quality and periapical status of the root-filled teeth were unsatisfactory in about half of the teeth. The predominant risk factors were high caries prevalence at age 10, dental anxiety and many missed dental appointments. Unrestored dentin lesions and deep dentin restorations were common and must be regarded as potential risk factors for endodontic treatment during adolescence.