ABSTRACT. We report a rare case of benign sex cord-stromal tumor consisted largely of luteoma with minor portion of Sertoli cell tumor located at the position of the left ovary excision in an 11-year-old ovariectomized bitch. Granulosa cell component was lacking, and both luteal and Sertoli cell portions were entirely positive for inhibin and neuron-specific enolase, whereas luteoma portion alone was positive for Wilms' tumor-1 (WT1), immunohistochemically. The results suggest that this tumor is a possible complication of incomplete ovarian excision at the time of ovariectomy and consisted of uncommon hybrid of luteal and Sertoli cells to be diagnosed as an unclassified sex cord-stromal tumor if applied in human cases. WT1-expression pattern suggested the signature of the difference in the phenotype of these cell types.KEY WORDS: inhibin , luteoma, Sertoli cell tumor, sex cord-stromal tumor, Wilms' tumor-1 (WT1).J. Vet. Med. Sci. 72(2): 229-234, 2010 In the old literature, a variety of types of sex cord-stromal tumors have been reported in bitches, such as granulosa cell tumor, Sertoli cell tumor, luteoma, and nonspecific stromal tumor [2,6,16]. In a later published retrospective study of 71 primary ovarian tumors in dogs, the incidence of epithelial tumors (46%) was slightly higher than that of sex cordstromal tumors (34%) or germ cell tumors (20%) [19]. Sex cord-stromal tumors were then equally divided into SertoliLeydig and granulosa cell tumors, and there was no indication of steroid cell tumors or luteomas. Granulosa cell tumors are usually the group of tumors that include granulosa theca cell tumors, and if they have abundance of lipid, they are diagnosed as luteinized granulosa cell tumors [30]. On the other hand, in humans, occasional tumors are encountered which clearly fall in the sex cord-stromal category but do not lend themselves to placement in the more specific categories, warranting the somewhat unsatisfactory diagnosis of sex cord-stromal tumor unclassified in both male and female gonads [25].This report describes a case of sex cord-stromal tumor with differentiation into luteal and Sertoli cells in an 11-year-old spayed bitch. At one year of age, this dog received ovariectomy for sterilization. At 10 years of age, bloody fluid appeared from valva. By ecography, one splenic tumor and another tumor located at the portion of the excised left ovary attached to the mesovaria were found, and they were then surgically resected. No other abdominal tumors/nodules were found at the surgery. The splenic tumor was histopathologically diagnosed as follicular lymphoma. The ovarian portion tumor consisted of a spherical and soft mass showing white and brown patchy pattern on cut surface. The mass was fixed in 10% neutral buffered formalin for three days at room temperature, trimmed, dehydrated, and paraffin embedded, and 4 m sections were stained with hematoxylin and eosin (HE). As immunohistochemical analyses, avidin-biotin-peroxidase complex (ABC) technique was applied using antibodies against inhibi...