Clinical and molecular virological differences were evaluated in 50 Japanese patients chronically infected with HBV of genotype B and C who were matched for age and sex as well as the severity of liver disease in a case-control study. Hepatitis B e antigen (HBeAg) was significantly less frequent (16% vs. 42%, P < .01), whereas antibody to HBeAg (anti-HBe) was significantly more common (84% vs. 56%, P < .01) in genotype B than C patients. The predominance of mutants with G-to-A mutation at nucleotide (nt) 1896 in the precore region (A1896) over the wild-type was comparable between genotype B and C patients (60% and 62%, respectively), and it correlated with anti-HBe. The double mutation in the basic core promoter (A-to-T at nt 1762 and G-to-A at nt 1764), however, was significantly more frequent in genotype C than B patients (58% vs. 16%, P < .01), and it did not correlate with anti-HBe or HBeAg. By the multiple logistic regression analysis, the double mutation in the basic core promoter (T1762/A1764) was significantly associated with genotype C [odds ratio (OR), 9.3; 95% confidence interval (CI), 3.4-25.1]], age ≥ 35 years (OR, 5.5; CI, 1.5-20.5), and more advanced liver disease (OR, 4.1; CI, 1.6-10.2), but it was not associated with sex, HBeAg, HBV DNA, or the precore mutation (A1896). These results suggest a role of the double mutation in the basic core promoter in association with genotype C and a longer duration of infection in the aggravation of chronic hepatitis B.