A 64-year-old man presented with symptoms of a lung infection. Radiologic work-up revealed opacity in the upper lobe of right lung. A computer tomographic (CT) guided fine-needle aspiration biopsy revealed sheets and clusters of mild to moderately atypical epithelial cells, small amount of mucin, histiocytes, multinucleated giant cells, and stromal fragments. The differential diagnosis included an inflammatory process and a well-differentiated adenocarcinoma. A lobectomy was performed and revealed abundant extracellular mucin surrounded partially by a thick fibrous wall, which was lined by neoplastic glandular cells. The histology was consistent with a pulmonary mucinous cystadenocarcinoma. Fibroblastic proliferation and foreign-body giant reaction secondary to extravasation of mucin were also noted. The patient was disease free 1 year after initial diagnosis. The relative lack of mucin as well as the presence of well-differentiated neoplastic cells, the inflammatory response to extravasated mcuin constitutes a potential pitfall in recognizing pulmonary mucinous cystadenocarcinoma cytologically.