The sequence variability in the pre-C region of the hepatitis B virus (HBV) genome in the serum of 42 anti-HBe antibody positive carriers with chronic hepatitis B was studied by PCR and direct sequencing to determine prevalence and type of HBV pre-C mutants in a highly endemic area. Except for one, all patients were infected with viruses containing mutations in the pre-C region which prevent precore and e-antigen (HBeAg) expression: 33 were infected predominantly or exclusively with variants containing a stop codon; two had a mixture of wild-type and a pre-C stop codon mutant virus; three had precore variants with mutations of the pre-C initiation codon and two of them an additional stop codon; four had a frameshift mutation; and one had two stop codons. One patient was infected with viruses which contained a mutation creating an amino acid exchange which should not prevent precore and HBeAg expression. These data demonstrate that in an endemic area a higher prevalence and even broader spectrum of pre-C HBV mutants are found than has been recognized previously in anti-HBe positive patients with chronic hepatitis B.