Pupil diameter was measured during the headache-free interval in 38 migraine sufferers selected from the general community. In each case, at least 70 percent of attacks recurred on the same side. Anisocoria was greater than in 40 control subjects, but miosis was not consistently greater on the usual side of headache. Average pupil diameter was similar in migraine sufferers and controls. In patients with pupillary dilatation lag on the usual side of headache, miosis persisted after 4% cocaine eyedrops. These findings suggest that cervical sympathetic outflow was lower on the usual side of headache in a subgroup of migraine sufferers. Pupillary dilatation to tyramine eyedrops was greater in control subjects than in migraine sufferers, consistent with decreased function of post-ganglionic cervical sympathetic fibres. Pupillary dilatation to 1% phenylephrine eyedrops did not differ consistently between the headache and headache-free sides, and was similar in migraine sufferers and controls. Thus, adrenergic supersensitivity of the pupils was not evident in this community sample of migraine sufferers. Vasodilatation or swelling of the arterial wall in the carotid canal could cause minor cervical sympathetic deficit in patients with frequent or severe attacks of migraine. Loss of sympathetic vascular tone could increase vasodilatation and pain during attacks.