Gliomatosis peritonei, a rare condition often associated with immature ovarian teratoma, is characterized by the presence of mature glial tissue in the peritoneum. We retrospectively evaluated 21 patients with gliomatosis peritonei and studied their clinicopathologic features and immunophenotype. The patients' ages ranged from 5 to 42 years (median, 19 years). Their primary ovarian tumors consisted of immature teratoma (n=14), mixed germ cell tumors (n=6), and mature teratoma with a carcinoid tumor (n=1). Gliomatosis peritonei was diagnosed at the same time as primary ovarian neoplasm in 16 patients and secondary surgery in 5 patients. Also, 11 of 21 patients had metastatic immature teratoma (n=4), metastatic mature teratoma (n=2), or both (n=5). One patient developed glioma arising from gliomatosis peritonei. Seventeen patients had follow-up information and were alive with no evidence of disease (n=13), alive with disease (n=3), or alive with an unknown disease status (n=1). The follow-up durations ranged from 1 to 229 months (mean, 49 months; median, 23 months). Immunohistochemistry results demonstrated that SOX2 was expressed in all cases of gliomatosis peritonei and glioma with tissue available (nine of nine cases), whereas OCT4 and NANOG were negative in all cases with available tissue (eight of eight cases). In conclusion, both gliomatosis peritonei and glioma arising from it show a SOX2+/OCT4-/NANOG- immunophenotype. These findings demonstrated that gliomatosis peritonei is associated with favorable prognosis, although it is important to rule out potentially associated immature teratoma and malignant transformation. SOX2 may have an important role in the development of gliomatosis peritonei.