Pulmonary histiocytosis X is an idiopathic benign disease characterized by proliferation and infiltration of lung tissue by characteristic Langerhans cells and eosinophils. Pulmonary histiocytosis X is common in young male adults, and shows variable clinical characteristics. We experienced a case of pathologically proven pulmonary histiocytosis X in a 30-year-old man who visit to our hospital due to chest discomfort and cough. The chest radiograph of our patient shows right pneumothorax and characteristic multiple thin-walled cysts on the both upper lung fields. The HRCT shows multiple thin-walled cysts, a few scattered nodules in both upper and right middle lung, and right pneumothorax.