To examine the progression of the proportion of hepatitis B virus (HBV) precore stop mutant (codon 28 of precore sequence; TGG to TAG) in the viral population of patients with dual hepatitis virus (B and C) infection, a series of serum samples obtained from six cases with chronic hepatitis B with acute hepatitis C superinfection were analysed by the method of amplification-created restriction site. At the time of superinfection, all patients included in the study were negative for hepatitis B e antigen and positive for its antibody. During the follow up of 30-90 months, in five of the six patients, three different patterns were observed: (i) the proportion of precore stop mutant increased in the first 20 months after acute hepatitis C virus superinfection and then decreased progressively thereafter (two cases); (ii) the proportion of precore stop mutant decreased progressively after superinfection (two cases); and (iii) no precore stop mutant was found during the course (one case). The remaining patient showed a progressive increase in the proportion of precore stop mutant over a period of 9 months after superinfection. Rather than the progressive increase in the proportion of precore stop mutants during the natural course of chronic HBV infection alone, the results of the present study showed a reverse course of progression following acute hepatitis C virus superinfection. This observation raised the possibility of a preferential or accumulative suppression on precore stop mutant by hepatitis C virus in the later stage of dual infection.