Transdermal clonidine has recently been reported to be efficacious in the prophylaxis of cluster headache. A 2-week course of transdermal clonidine (5 mg the first week, 7.5 mg the second week) prceded by a 5-day run-in period, was administered to 16 patients with episodic cluster headache in an active cluster period. In 5 patients, the painful attacks. disappeared after the seventh day of treatment. For the group as a whole, no significant variations in headache frequency. pain intensity, or attack duration were observed between the run-in period and the first and second weeks of treatment (ANOVA). Further studies are necessary to clarify the effectiveness of transdermal clonidine in the prophylaxis of episodic cluster headache.