A 4-year-old male, castrated, mixed-breed dog was presented to the Colorado State University Veterinary Teaching Hospital with a 1-week history of polyuria, polydipsia, lethargy, fever, inappetence, weight loss, and soft mucoid stool. The dog was depressed and had pale, icteric mucous membranes. Results of a CBC included normocytic, normochromic, nonregenerative anemia, neutropenia, and thrombocytopenia, with 43% blast cells (200/μL), many of which contained fine azurophilic granules. Cytologic evaluation of the bone marrow aspirates revealed mild granulocytic hyperplasia that appeared to be left-shifted in an apparent maturation arrest. A large population of blast cells comprised 35% of nucleated cells; the blasts had high nuclear to cytoplasmic ratios, deeply basophilic cytoplasm with vacuoles, and prominent nucleoli. Most cells also contained many fine azurophilic granules clustered in the paranuclear region. At necropsy, neoplastic cells were abundant in the bone marrow. Immunohistochemically the cells expressed CD3ε, and an oligoclonal T-cell rearrangement was found. The diagnosis was proliferative disorder of CD3+ granular lymphocytes, with associated pancytopenia. Because the blast cells were morphologically similar to myeloblasts and immunohistochemistry was required to confirm the diagnosis, T-cell lymphoproliferative disease should be considered in dogs with pancytopenia presenting with similar clinical features.