Twenty-seven endometrioid stromal sarcomas of the ovary from patients 38 to 76 (mean 56) years of age are reported. The tumors were unilateral in 20 cases and bilateral in 7. They were solid (9), solid and cystic (9), or predominantly cystic (6) when this information was known and ranged from 1 to 20 (mean 9.5) cm. The solid areas typically had a tan-yellow cut surface, with areas of hemorrhage and/or necrosis noted in 6; however, in addition, blood was often present in the cyst lumens. On microscopic examination, the predominant and frequently exclusive pattern was a diffuse growth of small cells with interspersed arterioles, the latter appearing round to elongated. A fibromatous pattern was present in 14 of the tumors but was extensive in only 3. A vague nodular growth was observed in 10 tumors but was never striking; a storiform growth was seen in 2 tumors, being conspicuous in 1. Hyaline plaques were present in 10 tumors but were striking in only 2. Sex cord-like or smooth muscle differentiation was seen in 7 and 6 tumors, respectively, being striking in 2 and 3 of them. Foam cells were present in 6 tumors. The tumors showed minimal cytologic atypia. The mitotic index ranged from <1 to 17/10 high-power fields (HPF), being <1/10 HPF in 12, 1 to 5/10 HPF in 9, 6 to 10/10 HPF in 2, and >10/10 HPF in 4 tumors. Infarct-type necrosis was noted in 12 tumors. Hemorrhage, typically recent, was seen in 20 cases, being conspicuous in 5. Ovarian endometriosis was intimately associated with the tumor in 16 cases. Seven patients had stage I tumors, 5 stage II, 13 stage III, and 2 stage IV. Follow-up information was available for 21 patients; 10 were alive and free of disease from 4 to 21 years postoperatively (follow-up being ≥ 11 y in 5); 6 were alive with disease from 1 to 22 years postoperatively; 5 patients are known to have died of disease, with the interval being unknown in 1, and 2, 4, 13, and 17 years in the others. Follow-up information was unavailable in the remaining 6 patients. These findings indicate that these tumors, as in the uterus, often have an indolent course with a better prognosis than other ovarian sarcomas, indicating the importance of correct diagnosis. The differential diagnosis of these neoplasms is in the first instance with a metastasis from the uterus; knowledge of the status of the uterus is paramount in this distinction. Associated ovarian endometriosis suggests a primary tumor. When a primary ovarian origin is determined, the differential diagnosis is most often with a sex cord-stromal tumor, particularly a granulosa cell tumor because of a diffuse growth of cells with scant cytoplasm.