Based on a preceding survey performed in 1985, the authors estimated the prevalence and incidence of cluster headache (CH) in the Republic of San Marino (26,628 inhabitants at 31 December 1999). All cases were diagnosed by direct interview according to International Headache Society criteria. The prevalence rate was 56/100,000 (95% CI 31.3 to 92.4), and the incidence rate was 2.5/100,000/year (95% CI 1.14 to 4.75). Most cases showed rare clusters. This is the first prospective study on the incidence of CH.