Aim: To determine the molecular basis of A(H1N1)pdm09 virus adaptation to a new host. Methods: The pandemic virus A/Russia/01/2009 was adapted to mice by serial lung-to-lung passages. The growth properties, pathogenicity and genome sequences of the mouse-adapted virus were compared with those of the parental strain. Results: The A(H1N1)pdm09 virus caused mouse death after only six lung-to-lung passages. Mutations in the influenza virus antigen were found in the brain, liver, kidney and in intestine by immunohistochemistry. Conclusion: Our findings suggest that multiple mutations in the viral genome promote rapid adaptation of the A(H1N1)pdm09 virus and cause generalized infection in mice.