We report a 22-year-old Peruvian-American woman with infectious mononucleosis who developed EB (Epstein-Barr) virus pneumonia. She presented with a 10-day history of fever and sore throat. Chest X-ray showed an infiltrative shadow in both lower lung fields, and she was therefore hospitalized. We suspected atypical pneumonia and administered quinolone as an antibacterial agent, but without effect. Blood tests revealed elevated lymphocytes and atypical lymphocytes on the hemogram. Real-time polymerase chain reaction for the blood EB virus was 2,000 copies/ml. EB virus-anti-VCA of IgM and IgG type were positive in the blood. We diagnosed infectious mononucleosis with EB virus infection. On the sixth hospital day, the infiltration worsened, and pleural effusion increased on chest X-ray. A bronchoscopy was performed, and EB virus DNA levels in bronchoalveolar lavage fluid (BALF) were 10,000 copies/ml. Typical lymphocytes increased in BALF. Pathological findings of the transbronchial lung biopsy specimens from the right B5 revealed lymphocytes positive for EB virus-encoded small nonpolyadenylated RNA (EBER) between alveoli. We diagnosed infectious mononucleosis with EB virus pneumonia. Clinical improvement occurred with supportive care. She was discharged on hospital day 26.