We describe 10 new cases and review 66 previously reported cases of primary pulmonary mucinous cystic neoplasia (PMCN). The 3 men and 7 women were 44 to 73 years old (mean, 60.0 years) at diagnosis. Lesions were found by chest radiograph (featuring a solitary, lobulated nodule with soft tissue density that enlarged slowly), or patients had major bronchial occlusion by mucus or hemoptysis. Tumors were well-circumscribed, lobulated soft masses with a central cavity filled with gray to greenish translucent mucus and were 1.5 to 5.5 cm in greatest dimension (mean, 3.3 cm). Microscopically, confluent lakes of mucin characterized all cases. Tumor epithelium ranged from bland to focal cytologic atypia to frankly malignant. The adjacent lung parenchyma was stretched, compressed, or showed an inflammatory reaction to dissected mucin. After 1- to 10-year follow-up (mean, 3.7 years), 3 patients died of metastasis and 1 of amitriptyline toxic effects; 6 were alive without tumor. Combined analysis of our cases and previously reported cases suggests a histologic spectrum from benign cystadenoma to mucinous cystic tumor with atypia to well-differentiated mucinous cystadenocarcinoma. The histomorphologic criteria derived from this analysis can help distinguish PMCN from other types of primary or metastatic mucinous tumors and predict outcome.