A self-administered questionnaire based on the operational diagnostic criteria of the International Headache Society (IHS) was evaluated in a cross-sectional epidemiological survey of headache disorders. A clinical interview was used as index of validity. Seven hundred and thirteen subjects were included. Sensitivity, specificity, predictive value, and chance-corrected agreement rate for the diagnosis of migraine was 51%, 92%, 50% (PVpos), 93% (PVneg), and 0.43 respectively. Corresponding values for episodic tension-type headache were 43%, 96%, 95% (PVpos), 46% (PVneg), and 0.30; and for chronic tension-type headache 14%, 100%, 100% (PVpos), 97% (PVneg), and 0.24. It is concluded that a questionnaire is not a satisfactory tool in diagnosing headache disorders according to the IHS criteria. The methods of data collection seem to have significant influence on the results. Caution in handling data obtained by means of subjective statements is advocated.