A liver transplant recipient reinfected with a lamivudine-resistant mixed wild-type/pre-S1-deleted hepatitis B virus (HBV) strain and rescued with adefovir dipivoxil was still HBV DNA positive after more than 1 year of therapy. Analysis of serum HBV DNA, amplified by polymerase chain reaction and directly sequenced by dideoxy nucleotide chain-termination method, showed that adefovir inhibited the wild type, but not the pre-S1-deleted HBV. Predominance of the pre-S1-deleted strain over wild type after adefovir treatment suggests that either adefovir inhibited the wild type more effectively or the pre-S1 mutant replicates more efficiently. The normality of liver condition confirms that to exert its pathogenic effect, the pre-S1-deleted strain requires the presence of wild-type virus.