MicroRNA-122 (miR-122) is abundant in the liver and binds to two sites in the 5′ UTR of the hepatitis C virus (HCV) genome, thereby protecting it from degradation and from the host immune response. Miravirsen is an antisense inhibitor that binds miR-122 with high affinity and arrests its activity. In this study, the efficacy and safety of miravirsen were evaluated in 36 patients with chronic HCV genotype 1 infection. The results show that the compound induces a dose-dependent reduction in HCV RNA levels that lasts for up to 14 weeks after the conclusion of treatment. Furthermore, no clinically significant adverse effects on renal function were detected, and there was no evidence for the emergence of viral resistance. Because miR-122-binding sites are conserved across all HCV genotypes and subtypes, targeting this host miRNA represents a promising strategy for anti-HCV therapy.