A 53-year-old man presented with cervical lymphadenopathy and massive splenomegaly. Peripheral blood examination showed many prolymphocytes with cytoplasmic azurophilic granules, giving an initial impression of large granular lymphocytosis. The lymph node biopsy and immunohistochemical study findings, however, were more compatible with a diagnosis of B-cell small lymphocytic lymphoma. The circulating prolymphocytes showed restricted kappa light chain expression similar to the lymphoid infiltrate in the lymph node. Karyotypic analysis revealed trisomy 15, a chromosomal abnormality that has rarely been described in small lymphocytic lymphoma or chronic lymphocytic leukemia.