Primary polycythemia (or erythrocytosis) is a chronic, myeloproliferative disorder that is characterized by clonal proliferation of noncommitted pluripotent or multipotent stem cells leading to an increased red blood cells (RBC) marked by an elevated packed cell volume (PCV), RBC count, and hemoglobin saturation. These changes occur independently of erythropoietin levels and the cause is unknown. A 12-year-old, neutered female, mixed breed dog was accompanied by diabetic control, with insulin therapy, during 2 years in our institution with no clinical signs of polyuria, polydipsia and weight loss and with laboratory findings between the reference ranges during this period. However, it was referred with a recent (1-month) history of lethargy and weight loss. On physical examination the dog was lethargic, had brick-red mucous membranes, and was 5% dehydrated; no other abnormalities were noted. Blood was submitted for a complete blood count (CBC), serum biochemistry analysis and ehrlichiosis serology. Abnormalities included markedly elevated PCV (72%) and RBC, normal cellular morphology, and hyperhemoglobinemia. The biochemical profile revealed a mild increase in alkaline phosphatase and serum glucose (230mg/dl) with no other abnormalities. Urinalysis disclosed a urine specific gravity of 1,035 with no signs of infection. Blood pressure measured indirectly using a Doppler ultrasonic transducer was 140 mm Hg systolic. Blood gas analysis from the right femoral artery revealed oxygen pressures of 93 mmHg and oxygen saturation of 97%. Thoracic and abdominal radiography, abdominal ultrasonography and Doppler echocardiography were unremarkable. Bone-marrow cytopathology and biopsy revealed erythroid hyperplasia with normal maturation and differentiation. The serum erythropoietin level was 2 mU/mL (range: 5 to 15 mU/mL). A diagnosis of primary polycythemia was made and phlebotomy (10ml/kg) was established. Hydroxyurea was initiated (15mg/kg body weight, per os (PO) q 48 hours until remission was achieved. Following phlebotomy and initiation of hydroxyurea therapy, the clinical signs of lethargy and weight loss resolved within 1-month and the PCV remains between 56%-58%. There was not necessary alter the insulin therapy regime and the dog remains clinically normal. Primary polycythemia occurs in humans, dogs, cats, horses, cattle and mice. It is rare in all species, with < 24 documented canine cases in the literature. There was no other case of primary polycythemia in association with diabetes mellitus in dogs until the moment.