We report a case of mullerianosis of the mesosalpinx, the appearance of which simulated metastatic adenocarcinoma. The patient was a 37-year-old woman with a mixed epithelial cystadenoma of borderline malignancy associated with endometriosis in both ovaries. The left mesosalpinx contained a firm, 2.2-cm, gray-white mass that on microscopic examination consisted of glands lined mostly by endocervical-type epithelium with admixed tubal-type glands and foci of endometriosis. Rupture of glands with extravasation of mucin into the surrounding stroma was observed, but there was no desmoplastic stromal reaction. Awareness of this relatively uncommon lesion is critical to avoid misdiagnosis and overly aggressive treatment.