The objective of this open single-centre study was to evaluate the efficacy and tolerability of botulinum toxin type-A (BTX-A) as add-on in the prophylactic treatment of cluster headache (CH). Twelve male patients with episodic (n=3) or chronic (n=9) CH, unresponsive to common prophylactic medications, were treated with a cumulative dose of 50 International Units (IU) BTX-A according to a standardised injection scheme into the ipsilateral pericranial muscles. One patient with chronic CH experienced a total cessation of attacks and in 2 patients attack intensity and frequency improved. In another patient with chronic CH typical attacks were not influenced, but an ipsilateral continuous occipital headache significantly improved. Patients with episodic CH did not benefit from BTX-A treatment. Tolerability was excellent. These findings provide evidence that BTX-A may be beneficial as an add-on prophylactic therapy for a limited number of patients with chronic CH.