Influenza virus infection is a major respiratory infectious disease that generally induces pneumonia. The clinical manifestations of influenza virus infection and community-acquired pneumonia (CAP) differ between elderly persons and younger adults. To determine the clinical features of influenza-associated pneumonia, we studied 21 adult patients with influenza-associated pneumonia, as indicated by positive test results for influenza virus antigen. At presentation, the higher-age patients (> or =75 years; n=12) with influenza-associated pneumonia had lower body temperature than did the lower-age (<75 years) patients (n=9). Conversely, the laboratory data indicated significantly higher C-reactive protein (CRP) concentration in higher-age patients than that in lower-age patients. None of the 18 patients undergoing neuraminidase inhibitor therapy died, but two of three patients who did not receive this therapy died from complications of advanced pneumonia. In this study, vaccination did not appear to be an important factor for prevention of pneumonia. High-age patients with CAP have lower body temperature, raising the possibility that diagnosis and treatment may be delayed in these patients. Increased CRP levels in these patients at presentation, however, could contribute to early detection of this serious pulmonary complication.