Mucoepidermoid tumors of lung (MET) are rare tumors derived from the minor salivary gland tissue of the proximal tracheobronchial tree. The authors studied 58 cases of MET confined to the lung and used criteria derived from similar tumors of the salivary glands to separate them into low-grade and high-grade variants. The overwhelming majority of low-grade tumors behaved in a benign fashion, whereas 23% of high-grade tumors resulted in patient death. Prognostic factors which appeared to predict future aggressive behavior included high-grade classification, advanced stage at presentation, and perhaps lymph node metastases.