Mucus-filled cystic tumors of low or borderline malignant potential, well recognized in ovary and appendix, have received little attention in the lung. We present data on 11 patients, all of whom had solitary pulmonary nodules resected in which mucus was the major histologic component. Prognosis appears good; no patient had developed local recurrence or metastatic spread of tumor (follow-up, 1 to 9.5 years; mean, 4.7 years). Columnar mucus-producing cells lined the cysts in all cases, with cytologic and architectural atypia varying from minimal to microscopic foci of carcinoma; paucicellular mucus dissection into surrounding lung analogous to pseudomyxoma peritonei was seen in seven cases. The histologic and clinical findings are consistent with a mucinous cystic tumor of low or borderline malignant potential.