H5N1 avian influenza virus is expanding its geographical distribution and human infections are being repeatedly detected in affected countries.Stockpiling antiviral drugs has been considered as a component of preparedness for a potential H5N 1 influenza pandemic.However, resistance to amantadine, an ion channel blocker, has been identified in the majority of isolates from some affected countries.While the stockpiling of the neuraminidase inhibitor oseltamivir has been started in many countries, viral resistance has been detected in some patients with H5N1 disease being treated with oseltamivir.Resistance to anti-influenza virus drugs is a matter of great concern and relevance for pandemic preparedness.