Precore defective HBV mutant has been suggested to be responsible for the pathogenesis of fulminant hepatitis. This variant is unable to produce HBeAg because of the presence of a novel translational stop codon at the end of the precore region of the genome. We examined the precore region of the HBV nucleotide sequence in 12 Japanese anti-HBe positive chronic hepatitis patients in order to ascertain whether the genome has stop-codon mutation in the precore region. Precore defective HBV mutant was found in most of the examined cases. A few cases had a mixture of strains, wild type and precore defective HBV mutant. The variety of precore genome concerned with the pathogenesis of chronic hepatitis still needs to be clarified.