We present the first prevalence study of specific headache entities using the operational diagnostic criteria of the International Headache Society. One thousand 25-64 year old men and women, who lived in the western part of Copenhagen County were randomly drawn from the Danish National Central Person Registry. All subjects were invited to a general health examination focusing on headache and including: a self-administered questionnaire concerning sociodemographic variables, a structured headache interview and a general physical and neurological examination. The participation rate was 76%. Information about 79% of the non-participants showed a slightly differing headache prevalence which was not quantitatively important. The following results in participants are therefore representative of the total sample. The lifetime prevalences of headache (including anybody with any form of headache), migraine, and tension-type headache were 93, 8 and 69% in men; and 99, 25 and 88% in women. The point prevalence of headache was 11% in men and 22% in women. Prevalence of migraine in the previous year was 6% in men and 15% in women and the corresponding prevalences of tension-type headache were 63 and 86%. Differences according to sex were significant with a male: female ratio of 1:3 in migraine, and 4:5 in tension-type headache. The prevalence of tension-type headache decreased with increasing age, whereas migraine showed no correlation to age within the studied age interval. Headache disorders are extremely prevalent and represent a major health problem, which merits increased attention.