The International Headache Society has published a system‘of classification and operational diagnostic criteria for headache based on clinical consensus.' This system is most useful for classifying patients in epidemiologic studies and clinical trials. Classifying headaches into primary (tension, migraine, or cluster) and secondary types (e.g., those caused by infection or vascular disease) is also useful to differentiate headaches that, although perhaps recurrent and temporarily disabling, have no dangerous underlying cause from those that may be a sign of significant pathology, because they represent an underlying systemic or neurologic disorder (Table l)