Eight definite and 10 possible cases of CPH are known to the authors. Decisive diagnostic features in the differential diagnosis versus ordinary cluster headache (Horton's headache) seem to be: the presence of headache every day, a high maximum daily attack frequency (greater than or equal to attacks/24 hours) and an absolute indomethacin effect. There is increasing evidence for a female preponderance in CPH. It emerges from this study that there frequently (or invariably?) is a pre-CPH stage with atypical attack pattern, usually lasting several years. Pregnancy seems to have a rather clear ameliorating effect on attack frequency and severity. In other patients, the very onset of headache is immediately after delivery. The importance of recognizing this special headache from a clinical point of view is clear since this disabling disorder can be completely abolished by drug therapy.