There have been a few reports of cluster headache with pain on one side and autonomic features on the other; sometimes the patient has previously had pain on the side of the persisting autonomic symptoms.3 Experiments with balloon catheters have shown that pain around and above the eye can be produced by stretching the internal carotid artery just below the syphon, a site where the artery is surrounded by the autonomic fibres supplying the eye.4 A sterile inflammatory response in the vessel wall might simultaneously narrow the lumen and press the nerve plexus against the bony skull, thus producing the watering eye and Homer's syndrome.5 6 Dilatation of the ipsilateral ophthalmic artery, without any changes in the internal carotid artery or circle of Willis, was seen during MRI in a patient who did not have a Homer's syndrome.7 The explanation for the apparent lack of correlation of the severity of the inflammatory process (as judged by the pain) with the severity of the autonomic dysfunction in this patient remains obscure. R C PEAT I-ELD Princess Margaret Migraine Clinic, Charing Cross Hospital, London W6 8RF