This study details the clinicopathologic features of 9 patients with endometrial stromatosis of the uterus, including 1 with the circumscribed variant. The potential of stromatosis for local invasion, intravascular penetration, and pulmonary metastasis is documented. Recurrent lesions often develop late, are amenable to surgical therapy, and do not necessarily portend a fatal outcome. Our findings and those of others indicate that the histopathologic appearance and relatively indolent behavior of stromatosis are sufficiently characteristic to justify its separation from the more aggressive homologous endometrial stromal sarcomas.