The objective of this study was to estimate the 1-year prevalence of the following categories of headache; migraine, non-migrainous headache, frequent headache (>6 days/month), and chronic headache (>14 days/month). Between 1995 and 1997, all 92,566 inhabitants 20 years and older in Nord-Trøndelag county in Norway were invited to a comprehensive health study. Out of 64,560 participants, a total of 51,383 subjects (80%) completed a headache questionnaire. The overall age-adjusted 1-year prevalence of headache was 38% (46% in women and 30% in men). The prevalence of migraine was 12% (16% in women and 8% in men), and for non-migrainous headache 26% (30% in women and 22% in men). For frequent headache (> 6 days per month) and for chronic headache (>14 days per month), the prevalence was 8% and 2%, respectively. Women had a higher prevalence than men in all age groups and for all headache categories. Prevalence peaked in the fourth decade of life for both men and women, except for 'frequent non-migrainous headache', which was nearly constant across all age groups in both genders. In accordance with findings in other western countries, we found that headache suffering, including migraine, was highly prevalent, especially in younger women.