From July, 2008 to March, 2009, 125 adults with community-acquired pneumonia (CAP) who were admitted to our hospital were retrospectively investigated to elucidate the characteristics of viral infection in adult CAP in Japan. Nasopharyngeal swabs for real-time polymerase chain reaction for 7 types of influenza virus, rhinovirus, respiratory synctial virus, human metapneumovirus, parainfluenza virus, coronavirus, and enterovirus were obtained. Diagnoses of viral infections were established according to positive results in real-time polymerase chain reaction and influenza rapid diagnostic testing, and based on a fourfold increase in antibody titer of influenza virus antibody in paired sera. Overall, a pathogen was identified in 74 patients (59.8%). Of these pathogens, 47 (37.6%) were bacterial, 17 (13.6%) were viral, and 10 (8.0%) were mixed virus and bacterial infection. Influenza virus (n = 12; 9.6%), rhinovirus (n = 8; 6.4%), respiratory syncytial virus (n = 8; 6.4%), and parainfluenza virus (n = 6; 4.8%) were detected. Adenovirus, coronavirus or enterovirus was not detected. Sore throat was more frequently found in patients with viral pneumonia than in those with non-viral pneumonia. Higher age and pneumococcal pneumonia were factors which contributed to severity in the present cases. It is difficult to distinguish viral pneumonia from non-viral pneumonia by clinical findings, and there were few clinically meaningful differences in presentation and severity, and no differences in severity or outcomes according to either the presence or absence of viral infection. Further studies are needed to clarify the possible significance of viral infection in CAP.