It has been suggested that the headaches which frequently precede or accompany transient cerebral ischemic attacks and completed strokes may be produced by dilatation of collateral circulation. This hypothesis was tested in studies of 400 patients in an acute stroke unit. Detailed clinical analysis was made of the headaches that occurred. Cerebral circulation was assessed by various combinations of dynamic technetium brain scans, contrast enhanced CT scans, cerebral angiography, and intracarotid 133 Xenon regional cerebral blood flow studies. Headaches occurred in 25% of the patients, and most had the clinical characteristics of vascular headaches. There was no correlation between headaches and cerebral hyperperfusion or collateral circulation. It is unlikely that simple dilatation of collateral circulation is a cause of the headache of ischemic cerebrovascular disease. Platelet aggregation with release of serotonin, prostaglandins, and other substances is a more probable cause.