The authors studied the occurrence of cluster headache in the families of 220 Italian patients with cluster headache. A positive family history was found in 20% (44/220) of the families. Compared with the general population, first-degree relatives had a 39-fold significantly increased risk of cluster headache. Second-degree relatives had an eightfold significantly increased risk. The increased familial risk strongly supports the hypothesis that cluster headache has a genetic component in some families.