We report on an extremely rare case of pulmonary mucinous cystadenocarcinoma. A 29-year-old male patient was admitted because of progressive enlargement of a right lower lobe mass over a period of 10 years. Right lower lobectomy was performed after a malignant mucinous cystadenocarcinoma was diagnosed by intraoperative frozen section. PET and CT scans did not detect metastatic disease. This case is the youngest patient reported so far with a malignant pulmonary mucinous cystadenocarcinoma and highlights the importance of close follow-up of indeterminate pulmonary nodules in patients with unremarkable history.