Adenomatous hyperplasia (AH), a hyperplastic parenchymal nodule, is detected most often through radiological and pathological observations in chronic advanced liver disease. AH can be classified into two categories: ordinary type or macroregenerative nodule (MRN) type I, and atypical type or MRN type II. Characteristically, both types contain portal tracts with portal veins and biliary elements. Ordinary AH may represent a large regenerative nodule with limited growth potential. Atypical AH shows slightly cellular and structural atypia, altered phenotypic expression, and increased proliferative activity compared to the surrounding liver. Histologic differentiation of atypical AH from well-differentiated hepatocellular carcinoma (HCC) is controversial, and generally accepted diagnostic criteria are still lacking. Atypical AH occasionally contains overt malignant foci. Histological observations and portography show that nodules of AH are supplied by portal venous blood. While AH nodules appear to be hypoperfused according to arteriographic studies, atypical AH contains small arterial branches which are abundant in contrast to the surrounding liver. The data obtained so far indicate that atypical AH may be a hepatocellular neoplasm which belongs nosologically to a gray area of neoplasm, in which overt HCC is likely to develop. It is clinically recommended that atypical AH is treated as "malignant" because of the occasional inclusion of HCC foci. For its treatment, percutaneous ethanol injection is becoming popular.