Twenty-two cases of primary malignant fibrous histiocytoma (MFH) of the lung are described, and a review of the literature is presented. As in the soft tissue, this sarcoma is found primarily in older individuals, usually as a solitary peripheral mass. Chest wall invasion at diagnosis was seen in four cases. The most common histologic subtype was the storiform-pleomorphic type of MFH. Vascular invasion was seen in 50% of cases. Histologic features did not correlate with prognosis, and subsequent aggressive behavior was related primarily to symptomatic presentation, advanced stage at diagnosis, incomplete excision, invasion of the chest wall or mediastinum, and subsequent recurrence or metastasis. Primary MFH of the lung should be distinguished from spindle cell carcinoma and inflammatory pseudotumors, and this histologic differential diagnosis is discussed.