We have assessed the validity and reliability of a self-administered headache questionnaire used in the 'Nord-Trøndelag Health Survey 1995-97 (HUNT)' in Norway, by blindly comparing questionnaire-based headache diagnoses with those made in a clinical interview of a sample of the participants. Restrictive questionnaire-based diagnostic criteria for migraine, assessed according to modified criteria of the International Headache Society, performed excellently in selecting 'definite' migraine patients (100% positive predictive value). The best agreement concerning migraine diagnoses was achieved by using a liberal set of criteria (kappa 0.59). Similar agreement was found evaluating patient status as headache sufferers, and as sufferers from frequent headaches (>6 days per month) (kappa 0.57 and 0.50, respectively). The kappa values of non-migrainous headache and chronic headache (> 14 days per month) were 0.43 and 0.44, respectively. The results suggest that our self-administered questionnaire may be suitable in identifying a population with 'definite' migraine, and the questionnaire is an acceptable instrument in determining the prevalence in Nord-Trøndelag of headache sufferers, migraine, non-migrainous headache, and frequent or chronic headache sufferers.