Cerebral blood flow (CBF) has been measured in 26 patients liable to cluster headaches. Of four patients studied during spontaneous attacks CBF fell in three and rose in one. Attacks could not be induced by nitroglycerine, alcohol or histamine except when patients were currently experiencing a bout of cluster headaches. The CBF rose during some induced attacks and fell during others. It is concluded that CBF changes do occur in cluster headache but the pattern is not consistent and does not at present suggest a mechanism for cluster headache.