Summary. Aim: Elevated body mass index (BMI) in chronic hepatitis C (CHC) has been associated with reduced rates of sustained virological response (SVR). The aims of this study were to determine whether early viral kinetics (and subsequently SVR) are influenced by weight or BMI by measuring HCV RNA at week 4 using two PCR assays with differing sensitivities. Methods: Patients with CHC treated with peginterferon plus weight-based ribavirin were included in this retrospective study. Body mass index, pretreatment viral load, genotype and liver histology were abstracted from the clinical database. HCV RNA PCR (lower limit of detection (LLD) <50 IU/mL) at treatment week 4 and 6 months after completion of therapy were recorded to determine the presence of rapid virological response (RVR-50) and SVR, respectively. In those who achieved RVR-50, stored week 4 serum was retested using Taqman (LLD < 15 IU/mL, RVR-15). Results: Of 134 patients included (genotype 1 57%, BMI 26.7 ± 4.5 kg/m2, ribavirin dose 13.9 ± 2.6 mg/kg/day), 59% achieved SVR. RVR-50 was observed in 39.6% and RVR-15 in 27.6%. Neither body weight nor BMI influenced RVR-50, RVR-15 or SVR. The positive predictive values (PPVs) of RVR-50 and RVR-15 for SVR were 88.7% and 97.3% (P = 0.23). RVR-50 and RVR-15 superceded genotype and viral load as the strongest independent predictors of SVR (OR 9.25 (1.9–45.11) and OR 30.74 (3.08–317.96), respectively). Conclusions: RVR is the strongest predictor of SVR. Early viral kinetics is not influenced by body weight or BMI when weight-based ribavirin is prescribed.