Gliomatosis peritonei (GP) describes the implantation of mature neuroglial tissue in the peritoneum and is usually associated with mature ovarian teratoma but is also found in cases of immature teratoma. We report the case of a patient with recurrent mature ovarian teratoma, GP, endometriosis (with malignant transformation), and carcinoid tumor, found at the time of hysterectomy for a primary endometrial adenocarcinoma. This unusual combination of tumor types has not been reported earlier. Metaplasia of the totipotential subcoelomic or submesothelial stem cells is a recognized pathway for the development of endometriosis. Evidence from molecular genetic studies suggests that a similar process of stem cell differentiation may explain at least some cases of GP. The coexistence (and colocalization) of endometriosis, GP, and carcinoid tumor in this case raises the possibility that peritoneal stem cells may occasionally show an even wider spectrum of aberrant differentiation. This has relevance for the assessment and management of patients with synchronous gynecologic tumors or presumed metastatic disease.