Neutropenia in rheumatoid arthritis (RA) is a problem that often needs to be addressed. Side effects of basic antirheumatic treatment, infections or substrate deficiencies are common causes; however, T‑cell large granular lymphocytic (T-LGL) leukemia, a mature T‑cell neoplasm, can also lead to autoimmune cytopenia. The T‑LGL leukemia can be associated not only with RA but also with other autoimmune diseases or neoplasms. Correspondingly, increases in clonal T cells, natural killer T (NKT) cells and LGL cells are found in the peripheral blood. A T‑cell receptor PCR and flow cytometry (or at least a blood smear) are therefore necessary to diagnose T‑LGL leukemia. The presence of clonal T cells alone is usually not pathological. A distinction must be made from Felty's syndrome (consisting of the clinical triad of arthritis, leukopenia, splenomegaly), which does not require the two T‑LGL leukemia criteria mentioned. The treatment for both entities (with underlying RA) is methotrexate and, if insufficiently effective, rituximab.