Sinusoidal “inclusion-containing endothelial cells” were studied histopathologically and immunohistochemically in various liver diseases, and their clinical importance was investigated. A total of 498 needle liver biopsies were examined. Endothelial inclusions inside the cells were recognized as eosinophilic granules in hematoxylin-eosin-stained sections. Electron microscopy showed that these inclusions corresponded to round cytoplasmic dense bodies with a single limiting membrane. The contents of these bodies were generally homogeneous, but sometimes heterogeneous. The inclusions appeared to contain protein, but were resistant to trypsin digestion, and immunohistochemistry failed to identify any immunoglobulins or hepatocyte-derived proteins. These endothelial cells also contained an increased number of micropinocytotic vesicles when compared with ordinary cells. The inclusion-containing endothelial cells appeared frequently in chronic hepatitis, but were relatively rare in other liver diseases. The incidence was higher in chronic aggressive hepatitis than in chronic persistent hepatitis or inactive cirrhosis. Although the density of these cells varied considerably even among patients with the same histological diagnosis and the phenotypical changes of these endothelial cells, assessed by monoclonal antibodies, were not apparent, the serum γ globulin level tended to increase in relation to the density of inclusion-containing endothelial cells and the correlation was significant in hepatitis C. (HEPATOLOGY 1994;20:604–610).