Introduction Mucinous cystadenomas of the ovary are clinically and histopathologically well-established and common tumors. Primary retroperitoneal mucinous cystadenomas are extremely rare. Such tumors are histologically similar to ovarian mucinous cystadenomas. Their histogenesis is still unclear. We report a case of primary retroperitoneal mucinous cystadenoma with foci of borderline malignancy containing a mural “sarcoma-like” nodule. Case report Clinical history 1 1 Fig. 1 arrowheads a b  Materials and methods The specimen was fixed in 4% buffered formalin. Representative samples were routinely processed and embedded in paraffin blocks. Four-micrometer-thick sections were stained with hematoxylin and eosin and with parallel routine immunohistochemical procedures. The antigens tested by immunohistochemistry were: pan-keratin, keratin Cam 5.2, cytokeratin 7, cytokeratin 10, cytokeratin 18, cytokeratin 20, epithelial membrane antigen, vimentin, desmin, actin, myosin, CD34, CD68, CD99, CD117, S-100 protein, and bcl-2. Pathological findings 2 Fig. 2 a b c d  2 2 1 Table 1 Immunohistochemical results of the sarcoma-like cells Antigen Result Pan-keratin − Keratin Cam 5.2 − Cytokeratin 7 − Cytokeratin 10 − Cytokeratin 18 − Cytokeratin 20 − Epithelial membrane antigen − Vimentin + Desmin − Actin +/− (some) Myosin − CD34 − CD68 + CD99 − CD117 − S-100 protein − bcl-2 +/− (some) Discussion Mucinous cystadenomas can be located in the ovaries, pancreas, and in the retroperitoneum. The mucinous cystadenoma presented was localized retroperitoneally near the pancreas but was clearly not adhered to it. Because normal-appearing ovaries were found, the cystadenoma was thought to be primary retroperitoneal. 9 9 2 4 9 18 2 2 14 14 3 9 5 8 10 17 Primary mucinous tumors of the retroperitoneum are very uncommon. These tumors can be classified into three clinicopathologic types: mucinous cystadenoma, mucinous cystic tumor of borderline malignancy, and mucinous cystadenocarcinoma. Our case was diagnosed as a primary retroperitoneal mucinous cystadenoma with only foci of borderline malignancy and a mural “sarcoma-like” nodule. 7 16 7 15 16 2 Table 2 Cases of primary retroperitoneal mucinous cystadenomas of borderline malignancy Study (year) Age (years) Sex Symptom Size, image Tumor marker Preoperative diagnosis History Pathology Extracystic extension Therapy Outcome 11 41 F Abdominal swelling, pain 12 × 10 × 9 cm UD UD UD MCAbor No TR UD 1 47 F Abdominal mass 10 cm (US) next to spl. ND Lt adrenal tumor App Hyst MCAbor No TR, resection spl. + lt adrenal NED 10 63 M Abdominal pain 6 cm (US), under rt kidney High CEA in cystic fluid Rt renal cyst NR MCAbor No ND NED 13 33 F Abdominal swelling, pain Large, unilocular lt (CT) ND ND NR MCAbor No LR NED, 10 months 12 33 F Abdominal mass 13 × 9 cm (US + CT) ND Mesenteric cyst NR MCAbor No TR NED, 12 months 2 48 F Abdominal fullness 15 × 13 × 9 cm (CT) ND Mesenteric cyst NR MCAbor No LR NED,8 months 6 41 F Flank pain, abdominal distension 21 × 16 cm rt (CT) ND Retroperitoneal cyst NR MCAbor No TR NED, 18 months 9 36 F Abdominal distension 12 × 8 cm rt (CT) CA 125:51 CA 19-9: 55 Ovarian cyst NR MCAbor No TR, App, Myo NED, 6 months Present case (2007) 45 F Abdominal pain 15 cm (US + CT) CEA, CA125, CA19-9: normal Cystic papillary adenocarcinoma or mucinous cystic neoplasm Endom MCAbor No TR NED, 12 months F M UD NR MCAbor TR LR NED US spl App Hyst Myo Endom rt lt