The purpose of our study was to identify general factors and distinctive clinical features differentiating patients with chronic cluster headache (CH) evolved from episodic CH and patients with episodic CH. Our study sample included 28 patients suffering from chronic CH evolved from episodic CH and 258 patients with episodic CH; all were referred to the Headache Center of Parma between December 1975 and June 1998. Patients with episodic CH were selected from all episodic CH referrals (n = 485) and selection was based on the duration of the disorder, which was to exceed the average period needed for an episodic form to turn into a chronic form (4.5 years for females and 7.0 years for males). At CH onset, the mean age for patients with chronic CH evolved from episodic CH was older than for those with episodic CH. Among patients with chronic CH, more were smokers or heavy drinkers, and had suffered a head injury. Clinically, episodic CH evolving into chronic CH was characterized by a high frequency of cluster periods, a larger proportion of patients with attacks not occurring strictly within cluster periods, and remission periods lasting less than 6 months. Possible predictive factors in the development of chronic CH appear to be CH onset from the third decade of life onward, the occurrence of more than one cluster period a year, and the short-lived duration of remission periods. The role played by head injury and cigarette smoking in the evolution of the disorder still cannot be established with certainty.