Migraine is considered to be a functional neurological disorder. For several years cerebral blood flow studies have been fueling the controversy surrounding the pathophysiology of migraine headache. Tc-99m HMPAO SPECT brain imaging was performed during the headache-free period in 44 migraineurs. The findings were compared with those of age 17 and sex-matched controls. The SPECT analysis was performed by using a 360 degrees rotating single head gamma camera system (Toshiba GCA 602A/SA, Japan), equipped with a LEAP collimator, interfaced to a Toshiba computer system, after 20 minutes following the injection of 350-550 MBq of Tc-99m HMPAO. The SPECT images revealed clear interhemispheric asymmetry in the upper frontal and occipital parts of the brain in migraineurs. It is suggested that an impaired regional cerebral vascular autoregulation may exist even during headache-free intervals in patients suffering from migraine.