T-cell large granular lymphocyte leukemia (T-LGLL) is a T-cell lymphoproliferative disorder that has recently been associated with B-cell dyscrasias on a spectrum ranging from dysgammaglobulinemia to lymphoma. To investigate the relationship between clonal B-cell and LGLL lymphoproliferations, we systematically studied lymphocytes in 57 patients with T-LGLL or NK lymphocytosis using flow cytometric methods sensitive to low-level B-cell populations. We identified 16 patients (28%) with abnormal B-cell populations; 9 (16%) of the patients had no known history of a B-cell lymphoproliferative disorder. We characterized these abnormal B-cell populations as monoclonal B-cell lymphocytosis and report a high frequency of monoclonal B-cell lymphocytosis in T/NK LGLL. Our findings suggest that certain pathologic factors may operate in patients with T/NK LGLL to drive low-level clonal B-cell proliferations.