Polycythemia is defined as an increase in hematocrit, in red blood cell count, and in hemoglobin concentration, and may cause severe clinical manifestations, secondary to hyperviscosity of the blood. Among these are: weakness, ataxia, disorientation, polyuria, epistaxis, hematemesis, and hematuria, among others. Primary, or vera, polycythemia, or primary erythrocytosis, is a chronic myeloproliferative disorder characterized by neoplastic proliferation of erythroid precursors with normal morphology, maturation and function. Such proliferation is independent of erythropoietin stimulation and does not respond to the regulating mechanisms, therefore the animals present normal or reduced erythropoietin content in their blood serum. It is considered a rare infirmity. We report the case of a spayed, 7 year old, female Weimaraner dog that presented at the Veterinary Hospital with weakness after physical exercise and prostration during the preceding 2 months. Physical examination revealed only congested mucosae. A complete blood count, biochemical analysis, thoracic radiographic, abdominal ultrasound and echocardiogram were performed. The blood count revealed only an elevated hematocrit (78%). No alterations were verified in the other requested exams. The myelogram revealed slight relative hypercellularity due to hypercellularity of the erythroid and megakaryocytic series. Erythropoietin dosage was < 1.00 mU/mL, considered lower than the reference values (1.8-12.0 mU/mL). The recommended treatment was an initial phlebotomy (20 mL/kg), with replacement of the same volume with saline solution, followed by administration of hydroxyurea at an initial dose of 30 mg/kg, every 24 hours, during 20 days. After this period, after a hematocrit of 56% was achieved, a maintenance period was initiated with a dosage of 15 mg/kg, maintained up to the present time. The animal has presented absolute clinical control, with periodic follow-ups, for the past year. There has been no relapse of the clinical manifestations during this period, nor have side effects or hematological alterations been recorded. The importance of this study is reporting a case of primary polycythemia and its successful treatment with hydroxyurea, which is accessible to most clinicians.