Primary carcinoma arising from a paratubal cyst in the mesosalpinx in uncommon. Serous tumors of low malignant potential outnumber invasive carcinomas, which are often of endometrioid type. Only five cases of serous papillary cystadenocarcinoma with capsular invasion have been documented. We report a case of invasive papillary cystadenocarcinoma arising in a large paratubal cyst of the mesosalpinx, in an infertile woman. Possible hormonal basis, its link to serous borderline and malignant tumors of the peritoneum, and value of pre/intra operative cyst fluid cytology are discussed. Lack of definitive management protocols, prognostic indicators and possible consequences are briefly reviewed.