Glypican-3 (GPC3), a member of heparan sulfate proteoglycans, plays a role in cell growth, differentiation, and migration. The objectives of this study were to assess the diagnostic value of GPC3 immunostaining in hepatocellular carcinomas (HCCs) and to analyze its expression profile in preneoplastic lesions. Tissue microarrays were built by sampling 54 HCCs and adjacent liver tissues (21 developing from cirrhosis and 33 from normal liver) and 94 cirrhotic macronodules. Fourteen typical liver cell adenomas and 5 with malignant foci were also included. Sections were assessed for GPC3 expression by immunohistochemistry. GPC3 staining was observed in 19 (90%) of 21 HCC cases with cirrhosis and in 18 (64%) of 28 HCC cases with normal liver (P < .01). When staining was positive, it was both membranous and cytoplasmic. Positive staining was observed in 1 case of nonneoplastic adjacent liver. In cases of adenomas, only malignant foci were positive. Among the 94 macronodules, GPC3 immunostaining was noted in 48% (14/29) of high-grade dysplastic or early HCC and in 3% (2/65, P < .001) of benign or low-grade dysplastic macronodules. This study shows that GPC3 is an efficient diagnostic marker of HCC, potentially useful in the differential diagnosis of liver cell adenomas and well-differentiated HCC. Our results also suggest that GPC3 may be considered as an early marker of liver carcinogenesis because it is able to identify some cirrhotic macronodules with malignant potential.