The cause of cluster headache remains to be determined. The involvement of peripheral neurovascular structures can explain the pain and autonomic signs of a cluster attack, but not its rhythmicity. The central theory of cluster headache attributes the cyclic recurrence to involvement of the hypothalamus. To evaluate hypothalamic dysfunction a number of hormone studies have been carried out on cluster headache patients. Alterations in plasma melatonin, cortisol, testosterone, gonadotrophins, prolactin, growth hormone and thyrotropin have been documented, some only in the cluster period but others in the remission phase of the illness. We believe that the hormonal abnormalities in cluster headache support disorders of hypothalamic function.