Chronic myelogenous leukemia (CML) is a neoplastic proliferation of neutrophil series, although concurrent eosinophilic and basophilic differentiation can occur. Neutrophils and neutrophilic precursors accumulate in the bone marrow and peripheral blood and invade other organs such as spleen, lymph nodes and liver. Total white blood cell (WBC) count is usually higher then 100,000 cells/μL, immature and mature neutrophils are present. Mature forms are more numerous, signs of dysplasia, hypersegmentation and giant forms may occur. The bone marrow is characterized by granulocytic hyperplasia without atypia and erythroid and megakaryocytic series may be affected, resulting in anemia, thrombocytopenia or, less commonly, thrombocytosis. Leukemoid reactions caused by inflammation or immune-mediated diseases must be distinguished from CML. We present the case of a five year old female Pit Bull, vaccinated and effectively dewormed, assisted for eight months with constant leukemoid reactions (88,000 to 147,000 WBC/μL), normal erythrocytes and platelets, without important clinical alterations. Clinical exam showed severe splenomegaly and articulate pain. Complete blood count (CBC) exam, serum biochemical profile, myelogram from bone marrow aspirate, urinalysis and lymph node cytology were performed. Left shift associated with leukemoid reactions (122,000 WBC/µL), polycythemia and normal platelets were observed. Bone marrow was hypercellular, with a predominance of immature cells, 5.2 myeloid: erythroid ratio and no cellular morphologic abnormalities. This result is compatible with CML, since no blast cells, inflammatory or infectious processes were present. Serum biochemical profile presented 170U/L of alanine aminotransferase, 2.02 mg/mL of bilirubin and 2.3mg/mL of creatinine, hyperproteinemia with hypergammaglobulinemia and urinalysis without alterations. Lymph node cytology showed large quantity of immature and mature neutrophils. Treatment with hydroxyurea (Hydrea^®) at the dose of 23mg/kg every 12 hours, for seven days followed by a new CBC exam. The treatment caused severe leucopenia, corpuscular volume (CV) of 23%, and 40,000/μL platelets, was interrupted for five days and support therapy was implemented. After the patients recovery, new CBC exam were performed, observing 150,000 WBC/µL, 32% CV, and 100,000/μL platelets, therefore chemotherapy was resumed with daily doses of 23mg/kg for two weeks. A next CBC exam showed reduction of WBC to 47,100/μL, 38% CV and 447,000/μL platelets. The dose was raised to 30mg/kg daily, this provided a WBC count reduction to 36,500/µL, 35% CV and 490,000/μL platelets, these values maintain unaltered to this date. The patient has been monthly monitored for about one year and has maintained WBC count in this range, with no important clinical or laboratory alterations.