To examine the phenotype of the sinusoidal endothelial cells (SECs) surrounding tumor cells and the process of capillarization in hepatocellular carcinoma (HCC), 51 primary HCCs, 4 adrenal metastases, and 3 portal tumor thrombi were immunohistochemically stained with monoclonal antibodies (MAbs) for CD4, CD14 (lipopolysaccharide-binding protein complex receptors), and CD32 (Fc gamma receptor II), which are specifically found on the SECs in normal liver, but not on ordinary vascular endothelial cells (ECs). Immunostaining was also performed for CD36 (thrombospondin receptors), EN4 antigen (Ag) (a pan-vascular endothelial cell Ag), PAL-E Ag (a venous and capillary EC Ag), factor VIII-related Ag (FVIIIRAg), and laminin. MAb 25F9, which identifies macrophages, was simultaneously used with the other MAbs to distinguish macrophages from SECs in HCCs (HCC SECs). CD4, CD14, and/or CD32 were found on HCC SECs only in 12 well-differentiated primary HCCs showing a thin trabecular or pseudoglandular tumor cell arrangement. These 12 tumors were smaller than those without CD4-, CD14-, and/or CD32-positive SECs (P < .05). Among them, 7, 5, and 11 tumors were negative or only partially positive for laminin, PAL-E Ag, and FVIIIRAg, respectively. Staining for laminin and PAL-E Ag showed an inverse relationship to the expression of CD4, CD14, and CD32 on HCC SECs and the tumor differentiation. In conclusion, the phenotypes of the SECs in early and well-differentiated HCC are thought to be similar to those of the SECs in normal liver. With progressing tumor dedifferentiation the HCC SECs lose the phenotypes peculiar to liver SECs and acquire the characteristics of capillary ECs, though both types of phenotypical change occur independently of each other.