A 16-year-old, Irish Draft mare was admitted to the referring veterinarian for an annual health check. A mild generalized lymphadenomegaly was noted. Rectal palpation and transrectal ultrasonographic examination revealed prominent mesenteric lymph nodes. A transcutaneous abdominal ultrasonographic evaluation was unremarkable. A CBC revealed a marked leukocytosis (63.06 × 103/μL) and lymphocytosis (58.2 × 103/μL) due to increased numbers of small lymphocytes. No evidence of anemia or thrombocytopenia was found and neutrophil counts were low-normal. Cytologic examination of fine-needle aspirates of multiple lymph nodes and a bone-marrow aspirate revealed the presence of a monomorphic population of small lymphocytes similar to those observed in the peripheral blood, suggesting a leukemic small cell lymphoma (SCL) or chronic lymphocytic leukemia (CLL). As the lymphadenomegaly and peripheral blood lymphocytosis were present simultaneously, the distinction between these 2 conditions was not possible. Immunophenotyping by immunocytochemistry and flow cytometry of the lymphoid cells in peripheral blood determined a T-cell phenotype. As the horse was clinically stable, no treatment was initiated, but regular examinations were undertaken. A CBC repeated 120 days after the diagnosis showed a marked lymphocytosis (157.6 × 103/μL) with no evidence of anemia or other cytopenias. The horse was euthanized 194 days after the initial diagnosis. Histopathology and immunohistochemistry of submandibular lymph nodes and bone marrow confirmed the diagnosis of leukemic SCL or CLL, and a T-cell phenotype. SCL and CLL are rare in horses; previous immunohistochemical studies determined that the T-cell phenotype is predominant. To the authors' knowledge, this is the first report of the combined use of immunocytochemistry and flow cytometry in a horse with leukemic SCL or CLL.