A malignant fibrous histiocytoma is a malignant soft tissue neoplasm that occurs frequently in the metaphyseal ends of the long bones of adults. The lung is a common site for metastasis but it is a rare site for a primary malignant fibrous histiocytoma. Here we report a case of a primary malignant fibrous histiocytoma of the lung. The patient was a 53-year-old man who presented with a moderate amount of a left pleural effusion and an illdefined mass in the left lower lobe on a chest radiograph and a local invasion to the left 10th and 11th rib on chest CT. Under the strong suspicion of lung cancer with a pleural invasion, a serial diagnostic thoracentesis was performed. The cytologic examination of the pleural effusion revealed no malignant cells. Consequently, a thoracoscopic pleural biopsy was performed. The histological examination revealed slender spindle cells and scattered epitheloid cells arranged in a vague storiform or a whirling pattern. Immunohistochemicaily, the tumor cells tested positive for vimentin and negative for cytokeratin, desmin, CD 34 and PAS. These features were consistent with a malignant fibrous histiocytoma. This case is an unusual addition to the small number of published reports on a primary malignant fibrous histiocytoma of the lung.