Some clinical as well as pharmacological indications seem to suggest that a reduction of the noradrenergic tone occurs in cluster headache (CH), during both the active and remission periods. But sharp fluctuations of the sympathetic system may trigger the attacks. Clonidine, an alpha-2-adrenergic presynaptic agonist, regulates the sympathetic tone in the central nervous system. Therefore, a continuous administration of low-dose clonidine could be potentially beneficial in the active phase of CH by antagonizing the variations in noradrenergic tone. After a run-in week, we administered transdermal clonidine (5 - 7.5 mg) for one week to 13 patients suffering from CH, either episodic (8 cases) or chronic (5 cases). During clonidine treatment, the mean weekly frequency of attacks dropped from 17.7 +/- 7.0 to 8.7 +/- 6.6 (p = 0.0005), the pain intensity of attacks measured on the visual analogue scale from 98.0 +/- 7.2 to 41.1 +/- 36.1 mm (p = 0.001), and the duration from 59.3 +/- 21.9 to 34.3 +/- 24.6 min (p = 0.02). This open pilot study strongly suggests that transdermal clonidine may be an effective drug in the preventive treatment of CH. Its efficacy is possibly due to its central sympatho-inhibition, which reduces or prevents the occurrence of fluctuations of noradrenaline release that may induce the attacks.