Hereditary haemorrhagic telangiectasia (HHT) is a rare autosomal dominant disorder characterized by a variety of abnormalities of the vasculature. This study investigated by questionnaire an unselected group of 58 patients suffering from HHT to identify and diagnose any headache history. These were compared with a control group comprising 40 patients with an unrelated autosomal dominant disorder, familial adenomatous polyposis. Fifty percent of the HHT patients reported attacks of headache which fulfilled current diagnostic criteria for migraine with aura. This was over four times the figure for the control group and at least 10 times the expected level for a normal population. The auras described by all the affected patients were classic visual disturbances preceding the headaches. Two patients also described typical visual auras unassociated with headache symptoms. The reason for such a clear association between migraine with aura and HHT is uncertain but cerebral arteriovenous malformations seem to be the likely link. A greater understanding of the reasons for the relationship may provide an insight into the pathophysiology of both conditions.