A 2.5-year-old spayed female American Pit Bull Terrier dog presented with a primary complaint of chronic refractory ascites. The dog's CBC displayed a moderate to severe macrocytic, hypochromic, nonregenerative anemia, and a moderate leukopenia as result of a moderate neutropenia and monocytopenia. Microscopic examination of the blood smear showed marked anisocytosis, mild polychromasia, mild acanthocytosis and ovalocytosis, moderate schistocytosis and poikilocytosis, and 4 metarubricytes/100 WBC. Abdominal ultrasonography revealed a homogenous, mild to moderately hyperechoic appearing liver as well as marked amounts of speckled anechoic to slightly hypoechoic peritoneal fluid. Cytology of the ascitic fluid demonstrated a sterile transudate, with evidence of a chronic inflammatory reaction as well as erythroid and myeloid precursor cells, and a few megakaryocytes with occasional micromegakaryocytes. Histologic sections of bone marrow, spleen, and liver were examined, using routine H&E stains, as well as a variety of immunohistochemistry and other special stains. Histopathology of the bone marrow and spleen revealed varying degrees of fibrosis, erythroid, and myeloid hyperplasia, as well as multiple small hyperplastic clusters of megakaryocytes. The megakaryocytes displayed many features of atypia such as increased cytoplasmic basophilia and occasional abnormal chromatin clumping with mitoses. Histopathologic examination of the liver disclosed evidence of mild extramedullary hematopoiesis. This case represents the first report of canine idiopathic myelofibrosis associated with peritoneal extramedullary hematopoiesis, resulting in refractory ascites. Although idiopathic myelofibrosis is a relatively rare condition in dogs, this case demonstrates that ascites caused by peritoneal implants of hematopoietic tissue may be the initial manifestation of myelofibrosis.