In 92 migraine patients and 44 healthy control subjects we recorded regional cerebral blood flow (rCBF) with single photon emission computerized tomography and133Xe inhalation or with i.v.99mTc‐HMPAO. Migraine patients were studied interictally. A quantitated analysis of right‐left asymmetry indices in a fixed set of regions of interest was compared with the normal asymmetry indices in the healthy controls. An asymmetry index deviating more than ± 2.5 S.D.s in normals was defined as pathological asymmetry. By quantitated analysis 47% of images from patients with aura attacks and 48% of images from patients without aura attacks were established to contain higher rate of asymmetries, the difference being statistically significant (p<0.05, Wilcoxon). A blinded visual analysis and scoring by a four level scale were done by four experienced observers. rCBF images from 18% of patients having attacks with aura and from 19% of patients without aura attacks was scored as containing abnormal right‐left asymmetries by the visual analysis. Images from healthy controls were all scored to be normal. In 37% of the images (all from patients) there was lack of consensus among observers (κ = 0.28). There was no correlation between visual or quantitated abnormalities and age, duration of migraine, frequency of attacks or prophylactic medication. No correlation could be established between asymmetries and the usual side of headache or aura symptoms. Two conclusions emerged: (1) visual evaluation of interictal migraine rCBF images is insufficient to pick up abnormalities; (2) almost 50% of the migraine sufferers had abnormal rCBF/asymmetries. However, these are discrete compared with those typically seen during the aura phase of a migraine attack. One explanation to the patchy rCBF patterns might be that they reflect interictal cerebrovascular dysregulation which might to be a common feature in both types of m