Three patients had leukocytosis of large granular lymphocytes and chronic neutropenia. Clonal chromosomal abnormalities (trisomy 8 and trisomy 14) and lymphocytic infiltration of splenic red pulp, hepatic sinusoids, and bone marrow indicated the neoplastic nature of the large granular lymphocytes. Demonstration of a T3+, T8+, HNK-1 + phenotype and low natural killer cell activity that was augmented by interferon treatment showed the leukemic cells to be immature natural killer cells. Multiple autoantibodies were present and included rheumatoid factor and antinuclear, antineutrophil, antiplatelet, and antierythrocyte antibodies, suggesting a defect of B-cell immunoregulation. In addition, in-vitro studies showed impaired suppression of immunoglobulin biosynthesis by abnormal cells from one patient. Antineutrophil antibodies and absence of direct cell-mediated inhibition of granulocyte-macrophage colony formation supported a humoral immune mechanism for the neutropenia. In these patients the syndrome of splenomegaly, multiple autoantibodies with neutropenia, and lymphocytosis of large granular lymphocytes is due to a neoplastic proliferation of immature natural killer cells.