The blink reflex was studied in 19 patients with migraine, 10 patients with tension-type headache, and 30 healthy controls. Significantly lower values of R2 and R2' amplitude and size were found in the migraine group, compared with the healthy control group. The differences were independent of the stimulation side (headache or nonheadache) and highly significant (P<0.001). The abnormalities of R2 and R2' amplitude and size were found only during the headache phase of migraine, being normal between migraine attacks. R1 latency and amplitude were normal in all patients. The blink reflex was normal in all the patients with tension-type headache. Subcutaneous injection of sumatriptan in 10 of the 19 migraineurs, during the headache phase, restored R2 and R2' amplitude and size values to normal. Our findings indicate that the brain stem interneuron part of the blink reflex arc may be diffusely suppressed in migraine, only during the headache phase. Furthermore, blink reflex may be an objective laboratory method to monitor the effectiveness of specific drugs proposed for the treatment of migraine.