We report four patients with expansion of a unique population of lymphocytes that is consistently associated with neutropenia. Two patients also had rheumatoid arthritis and autoantibodies. The lymphocytes contained many cytoplasmic azurophilic granules, which possessed strong acid phosphatase activity. Multiple cytoplasmic parallel tubular arrays were observed ultrastructurally. These granular lymphocytes showed the T suppressor/cytotoxic cell phenotype (E+, OKT3+, OKT8+, OKT4-, OKM1-, OKI1-) and exhibited antibody-dependent cell-mediated cytotoxic activity but little or no natural killer cytotoxicity. They did not respond to recall antigens, concanavalin A, or pokeweed mitogen, but the cells from one patient did respond to phytohemagglutinin. No in vitro suppressor cell activity on mitogenic responses of allogeneic cells and on mixed lymphocyte cultures could be demonstrated. There was no evidence of suppression of immunoglobulin synthesis in vivo. It is uncertain that the expansion of this subset of lymphocytes represents a leukemic process. Their constant association with neutropenia, however, raises the possibility that the increase in large granular lymphocytes and neutropenia might be pathogenetically related.