BACKGROUND The aim of this study was to assess the influence of hepatitis B virus (HBV) genotypes on serum hepatitis B surface antigen (HBsAg) kinetics in hepatitis B e antigen (HBeAg)-negative patients treated with pegylated interferon-alpha2a (PEG-IFN-alpha2a). METHODS A total of 48 consecutive patients treated with PEG-IFN-alpha2a (180 microg/week) for 48 weeks were assessed. HBV genotype was determined. Serum HBV DNA and HBsAg were assessed at baseline, during treatment (weeks 12, 24 and 48) and during follow-up (weeks 72 and 96). RESULTS The distribution of HBV genotype was A 27%, B 17%, C 12%, D 29% and E 14%. Mean +/-sd pretreatment serum HBV DNA (6.9 +/-1.5 log(10) copies/ml) was not different between genotypes and decreased under treatment in all genotypes without significant difference. Mean +/-sd pretreatment serum HBsAg (3.6 +/-0.6 log(10) IU/ml) was significantly different between genotypes (P<0.001), with high levels in genotypes A and C, intermediate levels in genotypes D and E, and low levels in genotype B (4.0 +/-0.3, 4.1 +/-0.7, 3.6 +/-0.5, 3.6 +/-0.4 and 2.7 +/-0.6 log(10) IU/ml, respectively). Serum HBsAg decreased under treatment in all genotypes with a significant difference. At the end of treatment, mean +/-sd decrease was high in genotype A, intermediate in genotypes B and D, and low in genotypes C and E (1.3 +/-1.6, 0.7 +/-0.7, 0.6 +/-0.9, 0.4 +/-1.0 and 0.4 +/-0.9 log(10) IU/ml, respectively; P<0.001). During follow-up, serum HBsAg continued to decrease in genotypes A and D, whereas rebound was observed in genotypes B, C and E. CONCLUSIONS HBV genotype has a strong influence on serum HBsAg kinetics during PEG-IFN-alpha2a therapy in HBeAg-negative patients.