Clinical manifestations and viral sequences of core promoter and precore/core region were compared among various genotypes of hepatitis B virus (HBV) in 25 patients with acute hepatitis. The genotype in patients with acute hepatitis was distributed differently from that among chronic hepatitis patients in Japan, which are predominantly genotypes B and C. Of 25 patients with acute hepatitis, 14 had genotype A, five genotype B and six genotype C. Serum total bilirubin levels were significantly higher in patients with genotype A than in those with genotype C. Prothrombin time was shorter in patients with genotype B than those with genotype A or C. Total bilirubin was lower in patients with short duration of acute hepatitis. The serum ALT value remained above 1000 IU/l for over 10 days in 79% of patients with genotype A. This prolonged duration of hepatitis in patients with genotype A may contribute to hyperbilirubinemia. Sequence analysis revealed no difference in the number of mutations in precore/core regions among the three genotypes. Although the double mutation, A-T and G-A at 1762 and 1764, respectively, was found in two patients each with genotype A and C, these mutations were not related to the hepatitis B e antibody (HbeAg)/hepatitis B e antibody (HbeAb) phenotype. Two of seven patients with thymidine at 1858 also had a G to A mutation at 1896. Thus, the difference in the genotype little influenced the HBeAg/HBeAb phenotype in acute hepatitis patients. Understanding the viral genotypes in acute HBV infection may be valuable in predicting the clinical course of acute hepatitis B (AHB).