Objective: To describe the use of dental floss by teenagers and study associations between flossing and approximal caries experience, oral health behaviours, gender, parental education and national background. Methods: The study included 2156 14-year-old teenagers. Data were collected in conjunction with routine dental examinations. Teenagers answered a questionnaire about frequency of and reasons for flossing, oral health behaviours and family characteristics. Information about approximal enamel (D1-2Sa) and dentine caries experience (D3MFSa) was collected from dental records. Data were cross-tabulated and tested with Chi-Square statistics and ANOVA, and analysed using multivariable logistic regression. The data was baseline data in a longitudinal study exploring effects of dental floss. The study was performed as part of the quality assurance system required by law in the dental services and did not require ethical approval. Results: Half of the teenagers (54%) used dental floss. Among teenagers who flossed, 15% reported doing so daily. Recommendation from dental personnel was the most important reason for using dental floss. Proportionately more girls and teenagers whose parents had high educational achievement reported flossing. Teenagers who flossed more often had more favourable oral health behaviours than other teenagers but more often had approximal enamel caries (D1-2Sa) and approximal dentine caries experience (D3MFSa) than other teenagers (p<0.05). Conclusion: Use of dental floss was not a daily behaviour in most teenagers. Flossing was associated with having approximal caries lesions, indicating that teenagers having signs of approximal caries had been recommended to use dental floss. Keywords: adolescent, caries, oral health behaviour, dental floss, topical fluoride.