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Although two classes of antiviral drugs, M2 ion channel blockers and neuraminidase (NA) inhibitors, are available to combat seasonal H1N1 and H3N2, and highly pathogenic avian (e.g., H5N1) influenza, the emergence and spread of drug-resistant viruses is of great concern. Animal studies suggest that combination chemotherapy with M2 and NA inhibitors may be an effective option to reduce the emergence of drug-resistant viruses. In this study, we evaluated the antiviral susceptibility of clinical isolates from immunocompetent children infected with a seasonal influenza A virus and treated with a combination of amantadine and oseltamivir. We found that amantadine-resistant viruses emerged during this combination treatment. While viruses with mutations known to confer oseltamivir resistance were not detected, we found viruses with mutations in NA that reduced sialidase activity and viruses with hemagglutinin mutations. These findings suggest that while the amantadine-oseltamivir combination is an effective therapeutic option for influenza, drug-resistant viruses do appear with this combination therapy. [ABSTRACT FROM AUTHOR]