A 9-year-old female spayed Labrador mixed breed dog was presented to the Cornell University Hospital for Animals Oncology Service for evaluation of a suspected leukemia. A marked lymphocytosis was incidentally discovered when the dog presented to the primary care veterinarian for a one-month recheck after an acute gastrointestinal upset. The dog had no clinical signs, but a complete blood count (CBC) showed that the lymphocyte count had increased from 1.7 thou/uL during the GI episode to 38 thou/uL (reference interval, 0.9-4.7 thou/uL). Small and intermediate sized lymphocytes were seen on review of the blood smear by the pathologist in the veterinary diagnostic laboratory, who suspected leukemia and recommended flow cytometry. A peripheral blood sample was sent out by the primary care veterinarian to Colorado State University for flow cytometry. The results showed a homogeneous expansion of small B cells, consistent with B cell CLL or lymphoma. On presentation, physical examination revealed firm, prominent mandibular and superficial cervical lymph nodes, mild splenomegaly and 3 cutaneous masses. Staging was performed prior to starting chemotherapy and included a CBC, chemistry panel, urinalysis, abdominal ultrasonography, and thoracic radiography. Further elevation of the lymphocyte count (51.3 thou/uL), mild non-regenerative anemia (HCT 38%), mild thrombocytopenia (152 thou/uL with no clumps), and a mild left shift without toxic change (band neutrophils, 4.5thou/uL) were found on the CBC. The chemistry panel showed hyperproteinemia characterized by hypoalbuminemia and hyperglobulinemia. On abdominal ultrasonography, there was splenomegaly with diffuse hypoechoic nodules and mild enlargement of several abdominal lymph nodes. The patient was started on oral cyclophosphamide and prednisone for presumptive CLL. This case report will discuss the challenge of differentiating CLL from certain types of lymphoma, with a focus on diagnostic testing, treatment options and prognosis in canine patients with CLL.