Pupil diameter was measured in darkness and in dull and bright illumination in 39 migrainous patients and in 15 tension headache sufferers during headache. In 21 migrainous patients, measurements were repeated during the headache-free interval. Mean pupil diameter was smaller in patients with common migraine at the time of examination than in 20 nonheadache control subjects, and smaller on the symptomatic side in migrainous patients with unilateral headache. During the headache-free interval mean pupil diameter did not differ from values in non-headache controls. These findings suggest that migraine is associated with a sympathetic pupillary deficit which is greater on the habitually-affected side.