Langerhans cell sarcoma (LCS) is a neoplastic proliferation of Langerhans cells that have overtly malignant cytologic features. It is a very rare disease and theoretically, it can present de novo or progress from an antecedent Langerhans cell histiocytosis (LCH). However, to our knowledge, LCS arising from an antecedent LCH has not been reported on. We present here a case of LCS arising from a pulmonary LCH. A 34 yr-old man who was a smoker, had a fever and a chronic cough. Computed tomographic (CT) scan revealed multiple tiny nodules in both lungs. The thoracoscopic lung biopsy revealed LCH. The patient quit smoking, but he received no other specific treatment. One year later, the follow up chest CT scan showed a 4 cm-sized mass in the left lower lobe of the lung. A lobectomy was then performed. Microscopic examination of the mass revealed an infiltrative proliferation of large cells that had malignant cytologic features. Immunohistochemical stains showed a strong reactivity for S-100 and CD68, and a focal reactivity for CD1a. We think this is the first case of LCS arising from LCH.