A population-based epidemiological study of the prevalence of headache in a sample of 24,682 adults was carried out in Finland. Enquiries were made concerning the frequency of headache, migraine and characteristics of headache attacks in a questionnaire survey in 1981. Headache was classified as migraine, and headache without migrainous features. Possible somatic causes of headache were excluded by record-linkage with three nation-wide medical registries. The prevalence of both headache types depended on age and gender. Among women the overall prevalence of migraine was 10.1% with a maximum of 11.5% in the 40-49 age group. The prevalence rate among men was 2.5% and the female/male gender ratio was 4.0. Women with migraine had an average of 52.5 headache occasions during a year and men with migraine 61 headache occasions per year. At the individual level, headache without migrainous features was least invalidating. There were only 16.5 headache occasions a year among women and 11.5 among men. However, from a population perspective headache without migrainous features accounted for 46.5% of all headache occasions among women and for 60.8% among men. Thus, from the public health view-point, studies of "non-clinical" headache, e.g. headache without migrainous features should be emphasized.