Ischemia is known to be a cause of hepatocellular apoptosis and atrophy in experimental animals, but the effect of vascular obstruction on such lesions in the normal or cirrhotic human liver has not been studied. The purpose of this study was to investigate the role of ischemia in the development of apoptosis, atrophy, and nodular hyperplasia in cirrhotic and noncirrhotic human livers. Apoptosis, focal atrophy, and nodular hyperplasia were semiquantitated in 203 liver specimens obtained at transplantation, segmental resection, or autopsy. These parameters were correlated with etiology, stage, activity, and acute and healed portal vein thrombosis (PVT). Large numbers of apoptotic cells were found in livers with acute PVT (17.2/medium-power field [MPF]) and in infarcts of Zahn caused by obstruction of portal veins (PVs) by tumor (16.4/MPF). Smaller numbers of apoptotic cells were found in cirrhosis of various etiologies (3.8-10.0/MPF) and rarely in normal livers (0.16/MPF). Evidence of healed PVT was found in 47% of cirrhotic livers and was associated with nodular hyperplasia (58% vs. 32%, P < .01) and focal atrophy (79% vs. 49%, P < .002). Apoptotic cells were found equally in those with and without healed PVT (40% vs. 38%, not significant). These observations suggest that apoptosis is a transient response to acute ischemia and that atrophy and nodular hyperplasia are chronic responses to ischemia. Vascular obstruction may be an important cause of the apoptosis and atrophy, which are found in nodular regenerative hyperplasia (NRH), infarct of Zahn, chronic hepatitis, and cirrhosis.