In Tokyo, from April to July 2009, seventy-four young people who live in the same dormitory presented at one municipal hospital with similar symptoms. Of these, 15 were diagnosed with pneumonia, and, sputum and blood samples were collected for culture. Adenovirus rapid antigen tests and adenovirus blood antibody tests were also carried out. One patient underwent a transbronchial lung biopsy. The demographic data and clinical features and the courses of all 15 pneumonia cases were also reviewed. The average patient's age was 25.5±3.74 (standard deviation). Streptococcus pneumoniae was detected by sputum culture in one sample, but nothing was detected in any other blood samples. Adenovirus rapid antigen tests were positive in 2 of 9 cases (22.2%). Blood levels of adenovirus type 11 antibody titer were elevated in 4 of 7 cases (57.1%). Histopathology showed smudge cell formation, and the test of adenovirus type 11 polymerase chain reactions was positive for lung tissue biopsy. All cases were resolved without specific treatments. We determined the outbreak of a pneumonia group infection by adenovirus type 11. Although pneumonia by adenovirus type 11 was thought to be very rare, an adenovirus antigen or antibody check should perhaps be carried out in cases of pneumonia group infection among young people.