Twin brothers with cluster headache are described. Monozygosity was demonstrated by conventional genetic markers and DNA-typing. Both had "mini-bouts" in the early stages. In the one, attacks were always excruciatingly severe; in the other, they started out as "mild", eventually becoming more severe. Both brothers also suffered from paroxysmal tachycardia. The connection between attacks of tachycardia and cluster headache remains enigmatic. The observation of cluster headache in monozygotic twins underscores the importance of genetic factors in the etiology. The ratio between cluster headache prevalence in close family members vs. prevalence in the general population may be higher in cluster headache than in migraine.