This study was performed to determine the distribution of hepatitis C virus (HCV) genotypes among asymptomatic carriers (ASC) and patients with chronic hepatitis without cirrhosis (NC- CH), liver cirrhosis (LC), and hepatocellular carcinoma (HCC) in the Osaka area, and to assess whether infection with HCV genotype l b (II) is more likely to develop into hepatocellular carcinoma (HCC) than is that with genotype 2a (III) or genotype 2b (IV). Genotypes of all study subjects were determined by Okamoto's method. HCV genotype 1 b was detected in 100 of the 143 ASC (69.9%), 551 of the 726 NC-CH patients (75.9%), 86 of the 103 patients with LC (83.5%), and 153 of the 179 HCC (85.5%) patients. Using unconditional logistic regression analysis, the age- and sex- adjusted odds ratios contrasting NC-CH with ASC, LC with ASC and HCC with ASC were 1.38 [95% confidence interval (CI) = 0.93-2.05], 2.28 (95% Cl = 1.12-4.63) and 2.27 (95% Cl = 1.02-5.06) respectively. HCV genotype l b is predominant in both healthy carriers and patients with chronic liver diseases in the Osaka area. The findings from the three case-control studies indicate that type l b infection is more closely associated with the development of LC and HCC than type 2a or 2b through its role in the progression of chronic liver inflammation to a cirrhotic stage. J Epidemiol, 1998 ; 8 : 244-249.