OBJECTIVES: Cost-effectiveness analyses of new hepatitis C drugs have been used to select and approve drugs that should be financed by the public healthcare system. However there are still a huge amount of uncertainties within these studies. The fact of having pharmacoeconomic and budget impact data of these new pharmacological alternatives will help to select the most efficient alternative. With this, the objective is to perform a budget impact analysis (BIA) of the treatment with pegylated interferon (pegIFN), alfa-2a or alfa-2b, plus ribavirin in, economically active genotype I patients, aging from 30 to 59 years, with chronic hepatitis C (CHC). METHODS: An interactive model has been designed from the inputs obtained from the medical literature. Both strategies have been considered as therapeutic equivalents, without significative difference in side effects, and as having the same price. RESULTS: The number of patients with CHC evaluated in the model has been of 355,611, 15.2% between 30 and 39 years, 13% between 40 and 49 years and 9% between 50 and 59 years, mimicking the Brazilian population and with an average weight of 70.6 kg. The duration of treatment was 48 weeks, with virologic response measured in weeks 2.4,12.24 and 48. A total of 91% and 99.8% of the patients receiving pegIFN alfa-2a or alfa-2b, respectively achieved sustained virological response at 72 weeks. Total cost was R$ 4,012,531,131.29 and R$3,752,238,973.99 for the treatment with pegIFN alfa-2a + RIB and pegIFN alfa-2b + RIB, respectively. CONCLUSIONS: Although cost effectiveness analysis appears to be favorable to pegIFN alfa-2a + RIB, a BIA seems to favour pegIFN alfa-2b + RIB due to its better predictability in the 12nd week of treatment. . In this case, the treatment with pegIFN alfa-2b + RIB (in comparison with pegIFN alfa-2a + RIB) is an efficient strategy.