Viral differences among lamivudine resistant hepatitis B (HBV) genotypes have not been yet investigated. Therefore, we analyzed the characteristics of these viral strains in vivo. Fortyone patients carrying lamivudine resistant HBV were enrolled. Twenty-six patients (63%) carried resistant HBV genotype A (group A) and 15 patients (37%) carried resistant HBV genotype D (group D). The rate of reverse transcriptase 204I mutants was significantly higher in group D (67%) compared with group A (19%), whereas rt204V mutants (81% in group A vs 33% in group D; P ؍ .006) and rt180M mutants (81% in group A vs 40% in group D, P ؍ .015) prevailed in group A. The median time of shift from rt204I to rt204V mutants was significantly shorter in group A (4 months in group A, >12 months in group D, P < .001). Additional resistance associated mutations were detected exclusively in group D (P ؍ .004). In a multivariate analysis, HBV genotype (P ؍ .039) and pretreatment serum HBV DNA (P ؍ .001) were independently associated with emerging rt204I or rt204V mutants, respectively. Serum HBV copy numbers after emergence of resistance were higher in group A (mean log 10 6.99 copies/ml; range 3-9) compared with group D (mean log 10 6.1 copies/ml; range 3.3-8; P ؍ .04). There was no difference between both groups regarding core promoter/precore mutations, viral turnover, and number of flares or disease progression during follow-up. In conclusion, the mutational pattern during selection of lamivudine resistant HBV strains differs between genotypes A and D. This may have consequences for a salvage regimen initiated for treatment of lamivudine resistant HBV. (HEPATOLOGY 2004;39: 42-50.) T he emergence of drug resistant hepatitis B virus (HBV) during lamivudine treatment for chronic hepatitis B is a major problem with an incidence of 14 -36% after 1 year of treatment. [1][2][3][4] This frequency increases to 38%, 49%, and 66% after 2, 3, and 4 years of treatment, respectively. [5][6][7] Lamivudine resistant HBV is characterized by amino acid variations in the reverse transcriptase domain of the HBV polymerase. In particular, an exchange of the methionine within the YMDD motif by an isoleucine or a valin (rtM204I/V mutants) is associated with lamivudine resistance. Breakthrough of these drug-resistant HBV mutants leads to a viral rebound to baseline levels, 8,9 to a decrease in the rate of loss of hepatitis B e antigen (HBeAg), 10 a high rate of relapses of serum alanine transaminase (ALT) levels, 11,12 and worsening liver histology. 13 Therefore, the emergence of viral resistance is one of the critical issues in the longterm outcome of patients treated for chronic hepatitis B. On the other hand, lamivudine resistant HBV is considered to have reduced viral fitness due to less replication efficiency in vitro 14 and lower ALT levels in vivo as compared with baseline levels. 4,15,16 This led to the recommendation to continue lamivudine treatment despite the emergence of resistant variants as long as benefit to the patient is maintained. 17 Taken together, it would be useful to identify factors which are associated with a better Abbreviations: HBV, hepatitis B virus; HBeAg, hepatitis B e antigen; ALT, serum alanine transaminase; PCR, polymerase chain reaction; CP, core promoter.