Immune-mediated hemolytic anemia leads to the premature removal of erythrocytes. It is a type 2 hypersensitivity reaction in which erythrocytes are covered by antibodies or complement and are phagocytized in the spleen or lysed within the blood vessels. It is considered of primary origin when the causes of the onset of the disease are unknown, or regarded as secondary when resulting from external antigens. The diagnosis may be therapeutic or based on the direct positive Coombs test in conjunction with laboratory findings, such as moderate to severe variable regeneration anemia, polychromasia, spherocytosis, red blood cell agglutination, hyperbilirubinemia, and hemoglobinuria. Treatment is based on immunosuppressive doses of steroidal anti-inflammatory drugs such as prednisone. This paper reports the case of an acute onset in a dog diagnosed with primary immune-mediated hemolytic anemia, presenting apathy, dyspnea, pain upon abdominal palpation, splenomegaly and pale mucous membranes. Coombs direct test was positive. Treatment started with prednisone, but as the response was not favorable, other drugs were associated to it. Evolution was unfavorable and the owner opted for euthanasia due to poor prognosis.