Increased body heat may precipitate cluster headaches, according to researchers from the City of London migraine clinic. They report a study of 200 patients in which a full history was taken during, or soon after, a cluster period (Lancet 1999;354:1001). Increased body heat as a result of exercise, a hot bath or the environment resulted in cluster headaches within one hour in 75 of the 200 patients. The authors comment that these headaches could be caused by generalised vasodilatation or hypothalamic activation. Avoiding factors that raise body heat may reduce the frequency and severity of attacks, they say.