Background: Pulmonary Langerhans Cell Histiocytosis ( PLCH) is a rare interstitial lung disease that occurs in approximately 10% of all LCH patients. The signs and symptoms of PLCH are nonspecific and characteristic radiographic finding is the most valuable clue for diagnosis. Eventhough an open lung biopsy is needed for definite diagnosis, it is invasive and can yield a non-specific result. Here, we report a case of PLCH which was diagnosed using bronchoalveolar larvage (BAL) cytology including immunohistochemical staining of CD1a. Case report: A 5-month-old girl was transferred due to incidental finding of a huge cystic mass in her left upper lobe (LUL) on chest x-ray and multiple cystic lesions on chest CT. She had neither respiratory symptoms, including cough, dyspnea, fever and weight loss nor abnormal finding on physical examination except for slightly decreased breathing sound on LUL. Outside chest CT finding was suspicious of PLCH and follow-up chest CT showed decreased size of multiple cystic lesions which suggests higher possibility of PLCH. Initial work-ups for LCH including bone marrow biopsy, whole-body magnetic resonance imaging, bone scan, skeletal survey and wedge resection lung biopsy were done. There was no evidence of LCH involvement in other organs. Furthermore, lung biopsy showed chronic inflammation and fibrosis and negative for immunostaining of CD1a. Thus, we planned 3-month interval follow-up with chest CT, skeletal survey without treatment. Her follow-up chest CT scan showed gradually decreased size and number of multiple cystic lesions and nodules in both lungs but increased extent of multifocal air trapping areas. For precise diagnosis, BAL was performed and BAL fluid analysis showed several large clusters of Langerhans cells, which consisted of 16 percent of CD1a (+) cells. She was diagnosed with PLCH and started on chemotherapy based on LCH-III regimen. Conclusions: BAL with immunohistochemical study can be a valuable modality to eliminate the possibility of infections and the other infiltrating disorders, and to diagnose PLCH for suspicious pulmonary lesions. Its sensitivity should be validated in a larger patient cohort.