A study was carried out on headache after carotid endarterectomy. A specific type of headache, similar in its characteristics to "cluster headache", occurred on the operative side in 30% of 54 patients, whereas no such headache occurred after extra-intracranial bypass or peripheral vascular surgery. This postoperative headache was not spontaneously reported by 56% of patients unless they were specifically asked about it. Pharmacological pupillary testing performed in 37 patients revealed that a decreased oculosympathetic activity (with or without adrenoceptor supersensitivity) was constantly associated with post-endarterectomy headache. Although this same abnormality was also observed in 54% of the patients without headache, a statistically significant (p less than 0.01) higher prevalence of decreased oculosympathetic responses was found in the patients with headache. The results suggest that damage to the sympathetic plexus due to the surgical procedure is involved in the development of postoperative "cluster-like" headache.