Mucoepidermoid carcinomas of the tracheobronchial tree are extremely uncommon and, as a result, opinions regarding their natural history are conflicting. In an effort to determine whether the tumors are aggressive or relatively benign, we have collected seven well-documented, previously unreported cases from among 4,250 primary pulmonary carcinomas and 116 bronchial adenomas. The two tracheal and five endobronchial lesions presented here include one high-grade and six low-grade tumors. Curative resections were performed, including segmental tracheal resections in two patients, lobectomy in three patients, and pneumonectomy in two patients, and the follow-up is complete to the time of this report. Long-term survivals ranging from 5 to 23 years, averaging 12.8 years, have been achieved in the six patients with a low-grade carcinoma. The one high-grade variant proved fatal within 28 months of diagnosis despite two surgical attempts at control and radiotherapy. It is concluded that these tumors exhibit a spectrum of virulence with low-grade lesions amenable to long-term surgical cure. The optimum treatment of high-grade lesions remains problematical.