Surgical specimens of four adult cases of cerebral pial arteriovenous malformation (AVM) were examined by reconstruction of serial sections to disclose the actual anatomical structure and to analyze the mechanism of gradual growth and rupture. More than seven arteriovenous shunts (AV-shunts) per case were found in all four cases, and these were divided into two types; one or a few shunts larger than 700 microm, and those with several shunts smaller than 700 microm in diameter. The former were located exclusively in the subarachnoid space and showed severe mural fibrosis. The vessels just ahead of and just behind these segments were also markedly involved by so-called arterialization of the vein and segmental loss of the internal elastic membrane (IEM), and/or smooth muscle cells (SMCs) and aneurysm formation of the artery. However, the smaller shunts were mainly localized in the cerebral parenchyma, where dilated small arteries showed abrupt loss of IEM and gradual loss of SMCs and transformed into dilated and tortuous veins, and the mural fibrosis was mostly absent at the shunts and segments ahead of and behind these shunts. It is suggested, on the basis of these findings, that AV-shunt of pial AVM occurs first in the subarachnoid space, probably prenatally, and gradually dilates along with developement of the brain and body, and that the subsequent abnormal burden of the blood inflow to the peripheral or related territory secondarily induces formation of new small AV-shunts mainly in the parenchyma. This secondary shunt formation may also be a component of growth of pial AVM in adults. Moreover, our reconstruction study suggested that hemorrhage of AVM is caused by rupture of not only dilated veins, but also intranidal microaneurysms and that thrombosis is not always necessary.