We investigated whether the criteria for idiopathic headache published by the International Headache Society (IHS) are useful in childhood and adolescence and compared the diagnoses according to this classification with those of Vahlquist. We used a semi-structured questionnaire to examine a total of 437 children and adolescents referred consecutively to a headache outpatient clinic. Twenty-eight of 437 patients were excluded because of symptomatic or unclassifiable headache. Of 409 patients with idiopathic headache, 70.4% had definite migraine or tension-type headache (IHS 1.1, 1.2, 2.1, 2.2), 20.5% had a migrainous disorder (IHS 1.7) and 9.1% had headache of the tension-type not fulfilling the criteria (IHS 2.3). In the differential diagnosis of migraine and tension-type headache the intensity of pain, aggravation of headache by physical activity, nausea and vomiting were the most important features. The quality of pain, photo- and phonophobia were less helpful and location least important. The duration of migraine attacks was less than 2 h in 19.0% of the migraine patients. In general, the diagnostic criteria of migraine were highly specific but less sensitive, and those of tension-type headache highly sensitive but less specific. The agreement between IHS criteria and those of Vahlquist was marked (kappa = 0.57). We conclude that the IHS criteria are useful for classifying headache in children and adolescents referred to a headache outpatient clinic. A forthcoming modification of the IHS criteria should consider a reduction of the minimum duration of migraine attacks from 2 h to 1 h and should try to increase the sensitivity of the criteria for migraine and the specificity of the criteria for tension-type headache.