||Objective Earlier epidemiological studies have shown that headaches are frequent among adolescents, especially girls. In particular, recurrent primary headache disorders such as migraine and tension-type headaches are common complaints in this age group. Headaches are increasingly being recognized as a significant health problem in adolescents and can lead to significant disabilities by affecting their lives, their school performance and their social lives. The aim of this study was to compare the prevalence of primary headaches among adolescents aged 16-20 years in Norway in two periods. Very few replicate studies have re-evaluated the prevalence of primary headaches in this age range and it is uncertain whether the prevalence is increasing. Methods Two cross-sectional, population-based studies were conducted in Norway from 1995 to 1997 (Young-HUNT 1) and from 1999 to 2001 (Young-HUNT 2). In Young-HUNT 1, 2594 adolescents in 2nd and 3rd grade in upper secondary school (aged 16-20 years) completed a comprehensive questionnaire including one question regarding headache during the last 12 months. In addition, 1730 of the students were interviewed about their headache complaints. In Young-HUNT 2, 2373 adolescents aged 16-20 years completed the same questionnaire and 1655 were interviewed in the same way as the earlier survey. The interviews were performed by trained nurses, and respondents were asked if they had experienced recurrent headache during the last year. If so, the headache was classified as migraine (MI), tensiontype headache (TTH) or non-classifiable headache (NCH). Headache frequency during the past year was recorded according to the following categories: Less than 1 day per month (less than monthly), 1-3 days per month (monthly), 1-5 days per week (weekly), or more than 5 days per week (daily). Results The participation rate was 88% in Young-HUNT 1 and 81% in Young-HUNT 2. The overall prevalence of having had headaches during the last 12 months did not change significantly (79.4% versus 77.5%; OR: 0.89, 95%CI: 0.79-1.02, p = 0.09), whereas the prevalence of recurrent headaches increased from 30.3% in Young-HUNT 1 to 35.4% in Young-HUNT 2 (OR: 1.26, 95%CI: 1.09-1.46, p = 0.002). The prevalence of tension-type headache changed significantly from 19.0% to 21.9% (OR: 1.20, 95%CI: 1.02-1.42, p = 0.03). Also the prevalence of migraine tended to increase (7.5% versus 8.7%, OR: 1.18, 95%CI: 0.92-1.52, p = 0.18). The overall frequency of recurrent headache changed towards more monthly and less weekly headache. Conclusions This is the first large-scale population-based study among adolescents in Norway assessing changes in the prevalence of primary headaches over a four-year period of time. The overall prevalence of recurrent headaches increased significantly from Young-HUNT 1 to Young-HUNT 2. An increase was seen in all types of recurrent headaches and was significant for tension-type headache. Implications Earlier follow-up studies have examined changes in prevalence of recurrent headaches in young age, but the results are diverging. Our findings support the impression that the prevalence of recurrent headache among adolescents is increasing, and underlines the need for prospective designed studies with emphasis on prognosis and etiological factors.