Cluster headache is classified as headache type 3.1 in the IHS Classification (Table 1). The features of cluster headache include: severe unilateral orbital, supraorbital, and /or temporal pain lasting 15 to 180 min untreated, conjunctival injection lacrimation, nasal congestion, runny nose, forehead and facial sweating, miosis (small pupil) , ptosis (drooping eyelid), eyelid edema. The frequency may range from one every other day to eight per day. The phrase "cluster headache" denotes a characteristic type of cephalgia defined as a severe unilateral head or facial pain, which lasts minutes to hours; it is often associated with ipsilateral lacrimation, nasal congestion, and facial flushing. The headaches tend to occur in separate bouts or clusters in one or more attacks daily for periods of weeks to months. Often during an attack, and sometimes persisting following the episode, there is ipsilateral miosis and ptosis, ie, and oculosympathetic paresis (Horner syndrome). The pain in cluster headache is very severe, being described as "boring," "sharp," "unbearable," and "the worst pain I have ever felt." The headache usually appears in or around one eye or on the cheek and then can spread to the temple, frontal region, occiput, or the ipsilateral neck. It rapidly builds to a peak within a few minutes, with an intensity greater than most other headache varieties. The usual duration is from 30 minutes to a few hours. Rather than lying down in seclusion, as preferred by most migraine sufferers, the cluster victim often paces about, holds onto his face, or applies very hot or cold water to the affected region, even to the point of injuring himself. The attacks usually occur once in a 24-hour period, often at a specific time, and frequently in the early morning hours awakening the patient from sleep. Men are affected much more commonly than women, in a ratio of approximately 5:1. Usually the episodes begin in the second or third decade. Typical migraine headaches may occur in the same individual with cluster, sometimes waning as the cluster commences. In the cluster attacks as described, it is extremely rare that any organic pathology is ever demonstrated; however, the variants of the syndrome require more detailed analysis. When the pain becomes persistent, becomes bilateral, or additional neurologic abnormalities are present (such as fifth nerve or optic nerve involvement), other conditions must be ruled out.