BACKGROUND: Currently, the approved dosage of ribavirin has not been studied in patients with 'normal' alanine aminotransferase (ALT) levels. METHODS: Modelling and simulations were performed using generalised additive models (GAMs) to predict the incidence of anaemia and rate of sustained virological response (SVR) in patients with hepatitis C virus (HCV) genotype 1 and persistently 'normal' ALT levels treated with peginterferon alpha-2a (40KD) 180 microg/week plus ribavirin 1000/1200 mg/day for 48 weeks. RESULTS: Model-based simulations predicted that SVR rates would increase from 39 to 48% if patients with genotype 1 and persistently 'normal' ALT levels had received the standard weight-adjusted dose of ribavirin. This was similar to the predicted 49% SVR rate for genotype 1 patients with elevated ALT levels. The incidence of anaemia was predicted to increase from 13% to 23% in patients with persistently 'normal' ALT activity and was higher than that predicted for patients with elevated ALT levels; however, the difference appeared to be largely explained by the higher proportion of women in the former group. CONCLUSIONS: Simulations based on GAM suggest that regimens for patients with HCV genotype 1 should include the standard weight-adjusted dose of ribavirin, as similar SVR rates are predicted to be achieved, regardless of patients' ALT status at baseline.