Endometrioid stromal sarcoma (also known as extrauterine endometrial stromal sarcoma [EESS]) is an uncommon tumor that occurs in women over a wide age range. The extrauterine location, non-gynecologic symptoms and signs at presentation, and confounding histologic features can pose a diagnostic challenge. In this study, we present the clinicopathologic features of 63 cases of EESS seen during a period of 21 years at our institution. Clinical information and pathology material were reviewed. Ages ranged from 27 to 87 years (median: 50 years). The most common symptoms and signs were an abdominal or pelvic mass, pain, vaginal bleeding, and gastrointestinal symptoms. The tumor size ranged from 1.2 to 24.5 cm. The most common sites of involvement were the ovaries (25), bowel wall (28), abdomen/peritoneum (37), pelvis (20), and vagina (6). Multiple sites were involved in 40 cases. Forty-six of 49 tumors had a classic microscopic appearance, and 3 had dedifferentiation; in 20 cases, there was vascular invasion. Fibroma-like stroma was seen in 30, hyaline plaques in 23, sex cord elements in 11, smooth muscle differentiation in 4, and myxoid change in 4 cases. Endometriosis was noted in 30 cases. Immunohistochemical results included: CD10 positivity in 31, desmin positivity in 9 (focal), estrogen receptor positivity in 28, and progesterone receptor positivity in 33 cases. In 25% of cases, an initial diagnosis other than EESS was made: sex cord-stromal tumors (4), gastrointestinal stromal tumor (3), leiomyosarcoma (3), liposarcoma (1), müllerian adenosarcoma (1), synovial sarcoma (1), malignant peripheral nerve sheath tumor (1), small round blue cell tumor (1), and atypical stromal endometriosis (1). Primary treatment was cytoreductive surgery for 61 patients and hormonal therapy for 2 patients. Adjuvant treatment included hormonal therapy, chemotherapy, and radiation therapy. Follow-up (5 to 336 months) information was available for 53 patients: alive with no evidence of disease, 29; alive with disease, 15; and dead of disease, 9 (median period of 70 months from diagnosis to death). Thirty-three patients had recurrent disease, and 10 patients were lost to follow-up. EESS is commonly associated with endometriosis and tends to be indolent with a propensity for recurrence. Seven of 9 patients who died of the disease had bowel involvement, and 3 had tumors with dedifferentiation. Besides the latter, no other histologic finding correlated with the clinical behavior of these tumors.