Benign mesothelial cystic inclusion is an uncommon lesion characterized by large, thin walled, multilocular mass. Most commonly it is found in relation to pelvic organs or genital tract, in association with a history of previous abdominal or pelvic surgery, endometriosis, or pelvic inflammatory disease. A mesothelial cyst included in Mullerian septum and its modification in pregnancy have not yet been described. A 35-year-old woman, gravida 2, para 1, had a medical history positive for a complete uterine septum and cervical duplication with a two simple cystic lesion (2.8 cm and 1.4 cm) inside the septum shown in 1997 (pelvic ultrasound examination, CT and MR). During her second pregnancy, pelvic ultrasound examination showed in the high right part of the pelvis a large bilocular, thin walled lesion, filled with ipoechoic non homogeneous substance. Pregnancy was uneventful but the mass increased dramatically. At 38 weeks of gestation the patient presented with dyspnoea. The cystic lesion measured 20 × 15 cm and filled the right side of the abdomen. A Cesarean section was performed. The operative appearance of the lesion was a bilocular cystic tumor, translucent, filled with mucinous fluid. Pathological examination demonstrated of cuboid mesothelial cell and filled with yellowish mucinous fluid, separated by fibrous tissue. Using immunohistochemistry the cells lining the cysts stained positive for calretinin and citokeratin. These findings supported the diagnosis of benign mesothelial cystic inclusion. No recurrence of lesion was noted on transvaginal ultrasound performed 6 months after surgery. In conclusion we hypothesize that complete uterine septum, that derives from a faulty resorption of the two parameso-nefric ducts covered by celomic epithelium, could contain mesothelial remnants which might preserve a capacity for growth and secretion.