Small lymphocytic lymphoma (SLL) is one of the subtypes of non-Hodgkin's lymphoma. Unlike the other lymphomas, SLL overlaps with CLL both clinically and morphologically.

Some practitioners consider SLL and CLL the same disease, differentiated only by how symptoms are presented in early stage disease. CLL shows up primarily in the bone marrow and peripheral blood, while SLL presents itself primarily in the lymph nodes or lymphoid tissues. Other practitioners see them as different diseases because of the different signs and symptoms that are associated with each. For example, a patient with SLL may complain of swollen nodes and of a low-grade fever while a patient with CLL might not have any complaints or a complaint of fatigue. A third group of practitioners visualizes these two diseases as the two ends of a straight line. They are connected and, as the disease progresses, come closer to each other, but they start out far apart.