A 15 year-old Persian queen, weighing 2,5 kg, was presented with complaints of discrete depression, anorexia and purulent vaginal discharge. The case history documented normal estrous cycles, with no breeding or treatment with progestogens. Abdominal palpation revealed a solid mass. An exploratory laparotomy was performed. A mass in uterine horn was detected, with absence of gross evidence of alteration in ovarian tissue. The ovariosalpingohysterectomy was performed. The patient recovered fully, and didn't have any sign of metastasis at the moment.

The resected uterus and both ovary were fixed in 10% neutral buffered formalin and embedded in paraffin. Sections were stained with hematoxylin-eosin and submitted to histological and immunohistochemical evaluation. Grossly, the intra-abdominal mass measured 2.0 x 2.0 cm, was grayish-white on the cut surface and firm. Microscopic evaluation of the uterus demonstrated multiple sites of ovoid and fusiform mesenchymal cell proliferation, atrophic endometrial glands, with deep smooth muscle infiltration. These cell exhibit moderate pleomorphism, basophilic cytoplasm, oval nucleus and moderate mitotic activity. The neoplastic cells expressed desmin, alpha-smooth muscle actin and CD10, compatible with endometrial stromal sarcoma tumor. Uterine neoplasms are seemingly rare in cats, and account for 0,29% of feline neoplasms. The majority of tumors related in feline uterus are adenocarcinomas and leiomyoma. Endometrial stromal tumors [ESS] are known to be rare in woman, only existing one report in cats; this is the second case of uterine ESS reported. In virtue of the rarity of this tumor, the behavior, breed and age predisposition, beyond metastatic potential are not known. The reported cat has been submitted to surgical procedure and at the moment of the report elaboration, didn't show any sign of metastasis. Although pyometra must be the first differential diagnosis, the tumors must also be considerate in differentials, mainly in old cats, with histologically and immunohistochemistry evaluation used as the most important tools for diagnosis and differentiation. In conclusion, uterine neoplasms in cats are rare, and the uterine tumor in the present case was an ESS. The present case was the second report of ESS (first report in South America), with any sign of metastasis and fully recovery after surgery.