Purpose: To investigate the possibility of influencing adolescents' caries incidence, knowledge and attitudes to oral health and tobacco through a school-based oral health intervention programme. Materials and Methods: Four schools in a county in Sweden were randomised to one intervention and one control group. At the intervention schools, two dental hygienists worked 4 h every week for two years. Health education and preventive measures, such as fluoride varnish treatments every 6 months, were performed. The control group had no intervention. The study included a total of 534 participants aged 12-16 years. Outcome variables of the study were 1) caries incidence assessed on bite-wing radiographs and 2) knowledge and attitudes measured through questionnaires. Results: The intervention programme impacted on the incidence of enamel caries, while no effect on dentin caries could be seen. Data from the questionnaires showed that the adolescents considered their teeth as important. Adolescents in the intervention group had better knowledge about oral health and oral hygiene compared to the control group after the two years, but no impact on attitudes toward tobacco could be seen. A majority of the adolescents were positive to the education about oral health and tobacco and they believed that the dental hygienists would help them improve their oral health. Conclusions: Although the effects of the intervention as regards to caries incidence, knowledge and attitudes were limited, the presence of dental hygienists in school was appreciated by the adolescents and seemed to increase their interest in their oral health. Keywords: adolescents, caries, fluoride varnish, oral health, prevention