We report a case of progressive dementia and prolonged gait disturbance correlated with influenza A/H3N2 infection in 91-year-old female patient, admitted because of in ability to take care of herself due to aging and cerebral infarction. At admission, conversation and comprehension were not significantly impaired, and she was able to walk by herself. Flu symptoms such as high grade fever, chills, arthralgia, and cough appeared after a short stay at home. Influenza A/H3N2 was confirmed serologically. Delirium occurred on the sixth day after influenza onset, persisted for three weeks, followed by recovery. Dementia symptoms such as memory defects and disorientation continued and did not improve. Due to this febrile episode, she was unable to walk unassisted. The results of computed tomography performed before and after the influenza episode were unremarkable for additional cellebro-vascular events during the observed period. Influenza infection may be an important risk factor for reducing the quality of life in the elderly. In geriatric cases, influenza should not be treated as a mere transient illness, but rather one which has important consequences for the elderly population, including the possibility of life threatening complications.