To examine the effects of polymorphisms in the gene encoding proinflammatory interleukin (IL)-1beta in patients infected with hepatitis C virus (HCV) in Japan, we studied 364 patients with chronic HCV infection (146 of whom had hepatocellular carcinoma [HCC] and 218 of whom did not) and 230 healthy control subjects. IL-1B-511 and IL-1RN genotypes were ascertained, and IL-1B-511 genotype T/T was found to be significant risk factors for the development of HCC, indicating that polymorphism in the IL-1B-511 genetic locus is one of the possible determinants of progression of hepatitis C to HCC.