A 17-yr-old female fallow deer presented with ataxia, inappetence, decreased fecal output, and decreased mentation. A complete blood count demonstrated leukocytosis (24.1 × 10³/μl, n = 1.16–7.38 × 10³/μl), characterized by lymphocytosis (22.89 × 10³/μl, n = 0.18–3.65 × 10³/μl), anemia (packed cell volume 20%, n = 29.0–55.8%), decreased red blood cell count (4.1 × 10³/μl, n = 6.86–14.72 × 10³/μl), and decreased hemoglobin (7.5 g/dl, n = 9.4–19.2 g/dl). Numerous mature, well-differentiated lymphocytes were noted on the blood film. Despite treatment and clinical improvement, the decision was made to euthanize the deer. Histopathology identified a monomorphic population of CD3 positive, CD79a negative small lymphocytes replacing most of the hematopoietic tissue in the bone marrow without evidence of tissue invasion. Results of viral screening were negative.