Rothia mucilaginosa is a gram-positive coccus of the family Micrococcaceae. R. mucilaginosa is considered a part of the normal flora of the human oropharynx and upper respiratory tract and lower respiratory tract infections attributable to R. mucilaginosa are not frequent. We present a case of pneumonia, in which the R. mucilaginosa infection was diagnosed by quantitative cultures of a bronchoalveolar lavage (BAL) specimen. A 46-yr-old woman with B lymphoblastic lymphoma was admitted to the hospital for scheduled chemotherapy. Her chest computed tomography (CT) scan revealed bilateral multifocal nodular and patchy consolidation in both lungs. Investigation of the BAL specimen revealed that 7% of leukocytes had intracellular gram-positive cocci. The quantitative cultures of the BAL specimen grew mucoid, non-hemolytic, and grayish convex colonies on blood agar at a count of approximately 200,000 colony-forming units/mL. The colonies were identified as R. mucilaginosa. The patient was empirically treated with levofloxacin for 7 days, after which findings on the chest radiograph and CT scan improved. She was discharged with improvement on hospital day 46. To our knowledge, this is the first report of R. mucilaginosa pneumonia diagnosed in Korea. Quantitative culture of BAL specimen and examination of intracellular organisms are crucial for assessing the clinical significance of R. mucilaginosa recovered from the lower respiratory tract.