Autoantibodies against insulin-like growth factor II mRNA-binding proteins (IMPs) were analyzed in patients with hepatocellular carcinoma (HCC) to elucidate the significance of these autoantibodies. Five of 86 (5.8%) HCC patients had one or more of these autoantibodies. Serum alpha-fetoprotein (AFP) levels ranged within normal limits in HCC patients seropositive for anti-IMPs except for one case. One of HCC patients had anti-IMP1 and anti-IMP3 before the diagnosis of HCC. On the other hand, overexpressions of IMP1 and IMP2 in the tumor tissues were observed in 2 (28.6%) and 3 (42.9%) of 7 HCC tissues, respectively. One HCC patient with IMP1/2-overexpression in the tumor tissue had anti-IMP1/2, while the other HCC patients with overexpressions of IMP1/2 in the tumors did not have anti-IMP1/2. These findings may suggest that autoantibodies against IMPs are produced in an antigen-driven immune system and that anti-IMPs seem to be supplementary serological markers for the diagnosis of HCC in AFP-negative cases or predictive markers of HCC.