Distinguishing a well-differentiated hepatocellular carcinoma (HCC) from normal and cirrhotic liver tissue or benign liver nodules, such as hepatic adenoma (HA) and focal nodular hyperplasia (FNH), may be very difficult in some cases, particularly in small needle core biopsies. We studied the expression of Glypican-3 (GPC3) and CD34 in 107 cases of HCC, 19 cases of HA, and 16 cases of focal nodular hyperplasia (FNH). In addition, we studied GPC3 expression in 225 cases of nonhepatic human tumors with epithelial differentiation. Ninety-four of 107 cases (88%) of HCC showed focal or diffuse cytoplasmic GPC3 staining, whereas all HA and FNH cases were GPC3-negative, and only 7 of 225 cases (3%) of nonhepatic tumors with epithelial differentiation expressed GPC3. The sensitivity and specificity of GPC3 for HCC was 88% and 97%, respectively. There were three CD34 staining patterns observed in hepatic tissue: negative, incomplete positive, and complete positive. In negative staining pattern, only blood vessels in portal triads or rare sinusoidal spaces immediately adjacent to portal tracts were positive. The negative staining pattern was seen in normal or cirrhotic liver tissue only. The complete CD34 staining pattern showed virtually all sinusoidal spaces with CD34-positive staining throughout the lesion. The complete CD34 staining pattern was seen in virtually all cases of HCC and in only some cases of HA and FNH. The incomplete CD34 staining pattern was characterized by either CD34 positivity in virtually all sinusoidal spaces in some but not all nodules or CD34 positivity in the peripheral sinusoidal spaces adjacent to portal triads. The incomplete CD34 staining pattern was seen in rare cases of HCC and in most cases of HA and FNH. We conclude that GPC3 is a very specific marker not only for differentiating HCC from nonhepatic tumors with epithelial differentiation, but also for differentiating HCC from HA and FNH. GPC3 immunoreactivity, in combination with a complete CD34 immunostaining pattern, greatly facilitates the accuracy of distinguishing between malignant hepatic lesions and benign mimickers.