Infectious mononucleosis usually develops when a patient is infected with the Epstein-Barr virus (EBV) for the first time. Its primary symptoms are fever, sore throat, and lymphadenopathy; pulmonary lesions are rare. A woman in her forties was admitted to our hospital with complaints of fever and general malaise. She was diagnosed with infectious mononucleosis, lymphadenopathy, hepatosplenomegaly, and liver dysfunction. She had atypical lymphocytes, and a serological test for EBV was positive. A chest X-ray revealed infiltrates in both lower lung fields, and chest computed tomography showed infiltrative shadows with atelectasis in the middle lobe of the right lung and the lingula of the left lung. We performed a bronchoscopy to diagnose the pulmonary lesions: transbronchial lung biopsy specimens confirmed lymphocytic infiltration into the alveolar septa and other pulmonary interstitial tissues, and cytologic analysis of bronchoalveolar lavage fluid revealed an elevated lymphocyte count and a low cluster of differentiation CD4/CD8. We diagnosed EBV pneumonia based on the presence of EBV-encoded small RNA (EBER)-positive cells. The pulmonary lesions fully resolved as the infectious mononucleosis improved.