Insulin-like Growth Factor 1 (IGF-1) antigen was immunohistochemically examined in 28 patients of the primary hepatocellular carcinoma with hepatectomy. IGF-1 was expressed in 93% (26/28) of the primary lesion and 100% (28/28) of the normal liver. Compared with expression in normal liver, decreased expressions in primary lesions were noted in 36% (10/28) for IGF-1. Histological examination revealed that there were significant correlations between patients with decreased expressions of IGF-1 in primary lesions and poor differentiated hepatocellular carcinoma, and portal vein infiltration. These results indicate that expression of IGF-1 has the relationship with the differentiation in human primary hepatocellular carcinoma.