In 1988 the International Headache Society (IHS) introduced new diagnostic criteria for headaches and craniofacial pain. Since headaches can be diagnosed solely on the basis of information provided by the patient, it is essential that the criteria are reproducible and consistent. Two neurologists evaluated the clinical records of 100 consecutive outpatients and transferred the data on headache and associated phenomena to a form designed to reflect the IHS criteria. Interobserver concordance (kappa statistics) in the application of the diagnostic criteria of primary headaches was: (i) "perfect" to "substantial" for the first IHS digit, being kappa = 1.0 for cluster headache and paroxysmal hemicrania; kappa = 0.88 for migraine; kappa = 0.75 for tension-type headache; (ii) "almost perfect" to "substantial" for the second digit (kappa = 0.94 for cluster headache; kappa = 0.90 for migraine with aura; kappa = 0.81 for episodic tension-type headache; kappa = 0.78 for migraine without aura; kappa = 0.71 for chronic tension-type headache; kappa = 0.66 for cluster headache-like disorder not fulfilling the criteria; (iii) "moderate" for migrainous disorder (kappa = 0.48) and headache of the tension-type (kappa = 0.43) not fulfilling the criteria. These results show that the IHS diagnostic criteria are satisfactorily applicable to high quality medical records abstracted by experienced neurologists.