Endometrial stromal sarcoma (ESS) involving the urinary bladder is very rare, with no prior series reported. We identified 6 cases of low-grade ESS involving the bladder at our institution (1998 to 2013), 5 of them consults. The median age at bladder involvement was 60 years (range, 44 to 77 y). One patient presented with bladder involvement at initial diagnosis of ESS. The remaining 5 cases with bladder involvement presented 7 to 30 years (mean 18 y) after a known diagnosis of ESS (n=2) or after a remote history of hysterectomy with an uncertain diagnosis (n=3). The location of bladder involvement included dome (n=1), trigone (n=2), diffuse (n=1), and unknown (n=2). Two cases demonstrated worm-like infiltrating tumor nests classic of low-grade ESS with little stromal reaction with retraction artifact mimicking vascular invasion. One case originating from the ovary showed focal glandular differentiation in the bladder, resembling endometriosis. Two cases had abundant keloidal collagen formation, arranged haphazardly or in a sunburst pattern. One case showed primitive cells infiltrating entirely hyalinized stroma, after chemotherapy given for a misdiagnosis of urothelial carcinoma. CD31 was negative in all cases, except for 1 case with obvious large vessel invasion. The differential diagnosis included a large nested variant of urothelial carcinoma, carcinoid tumor, synovial sarcoma, solitary fibrous tumor, Ewing sarcoma/primitive neuroectodermal tumors, and endometriosis. CD10 was strongly positive in 5 cases, and 1 case had very focal, moderate staining. Estrogen receptor showed strong and diffuse staining in all 6 cases. Progesterone receptor showed moderate to strong staining in 5 cases and focal staining in 1 case. One case showed PAX8 expression, and 2 cases showed p16 nuclear and cytoplasmic expression. CD56 showed weak to strong staining in 4 cases. Two cases had diffuse synaptophysin, and 1 case had focal p63 positivity. GATA-3, CD34, and CD99 were negative in all cases. The Ki-67 index was 1% to 10% (mean 4%). The mitotic count was 0 to 3/10 HPF (mean <1/10 HPF). Two patients had metastases to pelvic lymph nodes, and 1 had possible lung metastasis. Three patients were treated with Megace and 1 with Arimidex after surgery. Follow-up averaged 19 years (0 to 33 y) after the initial diagnosis of ESS or hysterectomy and 3.5 years (0 to 11 y) after bladder surgery. ESS involving the bladder is extremely rare with a very long interval from onset to bladder involvement. In female patients, low-grade spindle cell lesions involving the bladder should include ESS in the differential diagnosis.