A 6 kg, 9-year-old male Poodle was attended at Sena Madureira Veterinary Hospital with decreased appetite, weakness and disorientation for two months. Vomiting and seizures weren't reported. At physical examination, the dog was depressed and a large mass was palpable in the abdomen. The blood glucose level was 24mg/dl (reference range: 70-110 mg/dl). Ultrasonographic evaluation detected a heteroechoic mid-abdominal mass measuring 7,4 x 4.9 cm, but the origin wasn't identified. A fine needle aspiration of the mass along with cytology evaluation were inconclusive. Hypoglycemia was the only abnormality found on the serum biochemical profile. The serum level of insulin was 0,1 IU/ml (reference range: 5 to 20 IU/ml) with the serum glucose 21 mg/dl. The insulin/glucose rate supported hypoglycemia of nonislet-cell tumor origin. A noninvasive pedunculated mass originated from the jejunum was found during the exploratory surgery. The mass and a part of the Small Bowel were resected. The pancreas was normal. Visual or palpable metastasis wasn't noted on the liver or lymph nodes. The resolution of the hypoglycemia followed immediately after the mass was resected. A splenectomy was also performed, because small nodules were seen on the spleen. The final pathological diagnosis of the tumor was Small Intestinal Leiomyosarcoma and Hyperplasia Lymphoid of the Spleen. Neoplasms are an important cause of hypoglycemia in adult dogs. Insulinomas have been frequently described in dogs, and their diagnosis, staging, behavior, treatment and prognosis have been well characterized. Nonpancreatic tumors resulting in hypoglycemia have been infrequently reported in dogs. Nonislet-cell neoplasia in dogs is not associated with hyperinsulinemia, although low to low-normal insulin levels are usually observed. The cause of hypoglycemia in the dog reported is probably multifactorial: accelerated use of glucose by the tumor and insulin-like substances that may block insulin. The dog didn't have gastrointestinal symptoms as diarrhea or vomiting because the tumor was not invading the lumen of the intestine. Two months after the surgery, the dog had had a great recovery and blood glucose concentrations were within normal values. Leiomyosarcoma is a slow-growing, locally invasive, malignant tumor of smooth muscle origin that is typically slow to metastasis. Persistent hypoglycemia in an aged dog is not always due to an insulin secreting tumor. Prior to surgical exploration, careful physical examination, image exams and serum insulin measurement should be done to identify extrapancreatic tumors associated with hypoglycemia.