We have characterised the blood vessels found in normal cerebral vasculature and in arteriovenous malformations (AVMs), based on the expression of smooth muscle cell (SMC)-specific proteins. The marker proteins used were smooth muscle α-actin and four myosin heavy chain isoforms (SM1, SM2, SMemb and NMHC-A). Specimens of AVM obtained during surgery, and normal cerebral vessels from autopsy cases were studied immunohistochemically and compared. The arterial components of AVM contained an abundance of SMCs of the contractile phenotype, which were positive for α-actin, SM1 and SM2, but not for SMemb and NMHC-A. These components showed the same staining pattern as mature normal arteries. Two different types of abnormal veins were found in the AVM specimens: large veins with a thick and fibrous wall (so-called 'arterialised' veins) and intraparenchymal thin-walled sinusoidal veins. The former expressed α-actin, SM1, SM2, and SMemb, the latter expressed α-actin, SM1, and SM2. These marker expression patterns resembled those of normal cerebral arteries, and the results were compatible with arterialisation of the cerebral veins caused by arteriovenous shunting. However, the expression of SMemb was found only in the arterialised type of veins, not in the sinusoidal type or the arteries that had sustained abnormal blood flow in the AVMs. The thick-walled veins in the AVMs showed the same staining pattern as normal veins of dural plexus origin (large subarachnoid veins and dural sinuses). It is therefore possible to assume that they originated from the dural plexus, and extended into the brain during the formation of AVMs.