Extramedullary hematopoiesis (EMH) usually occurs in patients with severe anemia or myelofibrosis, and involvement of the serous cavities is uncommon. A total of 5 cases of peritoneal EMH are presented in patients presenting with primary gynecologic pathology including endometrial adenosarcoma (n=2), ovarian leiomyosarcoma, and ovarian endometrioid adenocarcinoma (each n=1), all of which were associated with peritoneal metastases; the remaining patient had a hemorrhagic benign ovarian cyst. All cases were associated with organizing peritoneal hemorrhage, and EMH was localized to the reactive granulation tissue. EMH was not identified within the tumor tissue in the 4 neoplastic cases. Erythroid precursors were present in all cases and granulocytic precursors and megakaryocytes were identified in two and three cases, respectively. There was no evidence of EMH in the corresponding peritoneal fluid cytology preparations examined in 4 cases. None of the patients had a significant hematological abnormality at the time of presentation or during a mean follow-up period of 35 mo (range, 2-66 mo). The mechanism of peritoneal EMH in these cases is uncertain but most likely related to tissue hemorrhage and repair as described in other sites such as dura, myocardium, and synovium. Pathologists should be aware that EMH may involve the peritoneum to avoid misinterpretation of the findings, particularly in small biopsy or cytology samples. © 2014 International Society of Gynecological Pathologists.