Abstract Mucinous cystic tumors of the lung are uncommon, the preoperative pathologic diagnosis is difficult and their biological behavior is still controversial. We report the case of a patient with a clinically benign cystic lesion that post-operatively showed to be consistent with an invasive adenocarcinoma arising in a mucinous cystadenoma of the lung, We underline the difficulty of the clinical pre-operative diagnosis of this cystic neoplasia radiologically mimicking a hydatid cyst, and we report the negative TTF1 immunostaining potentially misleading in the differential diagnosis with metastatic mucinous carcinomas. Finallly, we evidence the presence of a pre-existing mucinous benign lesion suggesting early and complete resection of benign appearing lung cysts because they can undergo malignant transformation if left untreated or they can already harbor foci of invasive carcinoma at the time of the presentation. Even if a good prognosis, better than in other lung carcinomas, with no recurrrence or metastasis after complete surgical exicision, has been reported for cystic mucinous cystoadenocarcinomas, the follow-up showed an aggressive biological behaviour, with the early onset of metastasis, in keeping with P53 positive immunostaining and high Ki-67 proliferation index.