Gliomatosis peritonei (GP) is the metastatic implantation of glial cells within the peritoneal cavity of patients with ovarian teratomas. The case of a young woman is presented, who initially developed a mature teratoma in the left ovary that was surgically removed. Nine years later a mature teratoma in the right ovary was excised, upon which GP was found in the greater omentum. To identify the cellular composition of the ovarian teratoma and of the omental implants, immunostainings were performed using antibodies against glial and neuronal antigens as well as against determinants of hematopoietic cells. In the teratoma the neuroectodermal part was strongly HNK-1-positive and contained GFAP- and vimentin-positive astrocytes and some NSE-positive neuron-like cells. In addition, neuroectodermal tissue was infiltrated by numerous CD68-positive macrophages/histiocytes and CD20-positive B lymphocytes. The omental nodules consisted of astrocytes, which expressed GFAP, vimentin and desmin. The implants also contained macrophages/histiocytes, which exhibited morphologic features reminiscent of microglial cells. In GP, macrophages might release glia-promoting trophic factors, which could allow the neural component of ovarian teratoma to implant in the peritoneal cavity and survive there for many years. Macrophage-derived factors might induce astroglial differentiation, which could explain why the peritoneal implants are mostly mature even when they originate from immature teratomas.