Mutations in the precore region of hepatitis B virus (HBV) have been associated with failure of expression of HBV e-antigen (HBeAg), however, the prevalence of these and other mutations in HBV carriers without overt chronic liver disease remains uncertain. Homosexual or bisexual males (n = 65) with chronic HBV infection attending The Middlesex Hospital, London were studied, of whom two had clinical evidence of chronic liver disease. HBV DNA was amplified from 62 of 65 serum samples using nested and double nested polymerase chain reaction (PCR) assays. Direct sequencing of the PCR products was employed to investigate sequence variation. HBV-DNA from all available HBeAg-negative (n = 9) and selected HBeAg-positive (n = 33) sera were sequenced in the entire precore gene, the 3' terminal portion of the X gene (aa128-154), and the 5' terminus of the core gene (aa18-73). Sequences were highly conserved in all regions studied. Samples from two anti-HBe-seropositive patients contained mutations in the precore region. In one, a single mutation in the first amino acid resulted in a change to leucine, which would prevent translation of this region and therefore HBeAg expression. Wild type sequences were also detected in this sample. In the other sample from a patient with overt chronic liver disease, a mutation of precore amino acid 28 changed a tryptophan residue to a stop codon which would also prevent HBeAg expression.(ABSTRACT TRUNCATED AT 250 WORDS)