Chronic lymphocytic leukemia (CLL) B cells are phenotypically identified by surface expression of CD5 and CD23 antigens. Infrequently, patients with a monoclonal B cell lymphocytosis clinically resembling classic B-CLL have been found to harbor leukemic B cells lacking expression of the CD5 antigen. Little information is available concerning such CLL-like lymphoproliferative syndromes. Here, we provide phenotypic and clinical characteristics of 13 patients with CD5-negative chronic lymphoproliferative disorders selected from among 400 B-CLL patients followed up at a single academic center. Phenotypic analysis was carried out by flow cytometry using a broad panel of monoclonal antibodies including activation, costimulatory, adhesion, and growth factor receptor molecules. Moreover, intracellular staining and stimulation experiments were performed to investigate whether CD5 antigen was either retained in the cytoplasm of clonal B cells or not expressed due to defective cellular activation, respectively. Overall, CD5-negative leukemic cells were found to express significantly different levels of several membrane molecules, including CD95, CD69, CD23, CD25, CD80, and CD20, compared to "classic" CLL B cells. CD5 antigen was not detected in the cytoplasm of CD5-negative clonal B cells, nor could it be induced following in vitro activation. CD3+ T cell proportions were found to be less affected in CD5-negative patients than in classic B-CLL. Although these data suggest that CD5-negative clonal B cells are phenotypically different from classic B-CLL, clinical outcomes were similar to those shown by B-CLL patients, with most of the patients experiencing a long-lasting disease requiring chemotherapeutic intervention at some time during the disease course.