A 26-year-old man was admitted to our hospital because of high fever, drowsiness, memory disturbance, and disorientation due to H1N1 influenza virus-associated encephalitis/encephalopathy. All of his symptoms rapidly improved following methylprednisolone pulse therapy. The diffusion-weighted image of brain magnetic resonance imaging (MRI) revealed a large transient hyperintense signal lesion on the central splenium of the corpus callosum. This MRI finding in conjunction with a complete clinical recovery has been previously observed in cases of clinically mild seasonal influenza-associated encephalitis/encephalopathy, and can be also a useful clue for the diagnosis of new type of influenza, H1N1 influenza virus infection complicated by encephalitis/encephalopathy.