Summary. Lamivudine resistance has been described in subjects with chronic hepatitis B infections, associated with mutations in the viral polymerase gene. The objective of this study was to estimate the emergence rate of lamivudine-resistant viral strains and their consequences over a 2-year period. We evaluated 283 lamivudine-naïve subjects with chronic hepatitis B. Clinical and virological features were assessed at inclusion and every 6 months thereafter. Viral DNA was characterized using polymerase chain reaction (PCR)-based sequencing. Potential risk factors for the emergence of lamivudine resistance mutations were assessed using logistic regression analysis. The annualized incidence rate for viral polymerase mutations was 22%. The only independent risk factor identified was high viral load, at inclusion. Detectable viral DNA and elevated transaminases were more frequent in subjects harbouring mutant viral strains, and these underwent a lower rate of hepatitis B e seroconversion. All subjects responded favourably to treatment, with no difference in symptoms between the two groups. This prospective cohort study identified lamivudine-resistant mutations emerging in 22% of subjects, yearly, which were apparently not associated with clinical aggravation over the study period.