Background: In dogs, abdominal testes have higher risk of malignant transformation, and the Sertoli cell tumor is the most common type of testicular cancer in the species. This neoplasia can lead to an increase on estrogen production, causing a paraneoplastic feminization syndrome with dermatologic, behavioral and blood repercussions. When located inside theabdomen, the Sertoli cell tumor can compress adjacent organs, worsening the prognosis. Thus, this study aimed to report a case of an intra-abdominal Sertoli cell tumor in a dog associated to multiple organic complications.Case: A 4-year-old male Shih-tzu was admitted at the Veterinary Hospital of Universidade Federal Rural do Semi-Árido (UFERSA), with a history of abdominal cryptorchidism and the development of dermatological signs, such as alopecia and hyperpigmentation, with an evolution of two months. At physical examination it was observed alopecia on lumbosacral regionand on hind limbs, hyperpigmentation, gynecomastia and pendulous prepuce, suggestive of feminization syndrome. A firm mass was palpated on caudal abdomen. The following exams were requested: complete blood cell count, serum biochemistry (alanine aminotransferase, alkaline phosphatase, creatinine and urea), abdominal ultrasonography and fine needle aspiration cytology of the abdominal mass. Hematological and biochemistry profile were within the normal range for the species. Onthe ultrasonography, regular and rounded structures were observed, suggesting ectopic testes (left testis: 2.37 x 2.95 cm; right testis: 1.92 x 1.38 cm). The left testis presented characteristics of malignant transformation. The cytology of the abdominal mass suggested Sertoli cell tumor. The dog was submitted to an exploratory laparotomy. The right testis was atrophied and its orchiectomy was performed. The contralateral testis was completed adhered to great vessels, such as abdominal aorta and caudal vena cava, and also to adjacent viscera. Thus, the exeresis of the tumor was not possible and fragments of the mass were collected and referred to histopathology. The left testicle presented tumoral polyedric cells, alonged at the longitudinal axis, propagating disorderly, and with central rounded nucleus, and a clear and vacuolar cytoplasm. It was alsoobserved desmoplasia, anisocytosis, anisocariasis, nuclear atypia and evident nucleoli. The mitotic index was less than one mitotic figure/40x, with aberrant mitotic figures. The histopathological evaluation was compatible with Sertoli cell tumor, confirming the presumptive diagnosis. Chemotherapy was proposed, but the animal’s owner refused. After four months the dog was readmitted presenting a series of complications due to the tumor growth, including bone marrow hypoplasia, gallbladder compression and hydronephrosis. Because of the poor prognosis and severity of complications a decision to euthanize the animal was made.Discussion: Intra-abdominal Sertoli cell tumors have good prognosis when the diagnosis and surgical treatment are made early. Nevertheless, this neoplasia can be associated to critical and irreversible damages to various organs, such as bone marrow hypoplasia, and compression of abdominal organs when the tumor grows rapidly and invades adjacent structures,making the complete surgical exeresis impossible. Therefore, the orchiectomy of cryptorchidic dogs is recommended as soon as possible to prevent the development of testicular cancer in these animals.Keywords: cryptorchidism, testicular neoplasia, Canis lupus familiaris.