A 2-year-old female mongrel dog was referred to the Veterinary Medical Center of Chungbuk National University due to anorexia, depression and weight loss after the administration of exogenous estrogen for contraception in the local hospital. Although the dog was medicated with antibiotics and fluid for two weeks in the local hospital, there was a deterioration in the patient`s general condition. Therefore the dog was referred to our Veterinary Medical Center. Hematological and biochemical analysis showed non-regenerative anemia, thrombocytopenia, severely immature leukocytosis, hyperbilirubinemia, hyperglycemia and elevated serum alkaline phosphatase, gamma-glutamyltransferase, and lactate dehydrogenase. Abdominal ultrasonography showed double line of gallbladder and irregular hypertrophic wall of uterus. On the basis of these examination`s results and history taking, final diagnosis was exogenous estrogen-induced myelotoxicosis with cholecystitis and endometritis. The symptomatic therapy for estrogen toxicosis was performed for 26 days after the final diagnosis. However, the symptom had no response to therapy except endometritis and anemia. On day 135 after administration of estrogen, platelet and neutrophil count were normalized, but monocytic leukocytosis was occurred.