['HCV has a high replication rate and a lack of proofreading activity, leading to a greatly diverse viral population. This diversity may lead to emergence of resistant strains in direct-acting antiviral therapy. The frequency of naturally occurring HCV protease inhibitor (PI) mutations has been addressed in many countries, but there are few data on the prevalence of these mutations in Brazilian patients.', 'We evaluated the sequence of HCV NS3 protease gene in 247 patients (135 HCV-monoinfected and 112 HIV-HCV-coinfected patients). HCV RNA was extracted from plasma and a fragment of 765 base pairs from the NS3 region was amplified and sequenced with Sanger-based technology.', 'HIV-HCV-coinfected patients were more likely to be older than 40 years and have an HCV subtype-1a infection. Overall, 21.9% of patients had at least one amino acid substitution in the NS3 region; 14 patients (5.7%) harboured at least one resistance mutation (T54S, V55A, Q80R) and the Q80K mutation was not found in our case series. There was no difference between monoinfected and coinfected patients regarding the frequency of natural polymorphisms and resistance mutations.'] Are resistance mutations rare among protease inhibitor treatment-naive hepatitis C genotype-1 patients with or without HIV coinfection?