We describe a patient with neutropenia associated with increased circulating large granular lymphocytes (LGL). Absolute neutropenia was accompanied by the absence of myeloid precursor cells in the bone marrow. No myeloid progenitor cells (CFU‐C) could be detected by in vitro colony culture. The peripheral blood was also remarkable for the presence of a population of large granular lymphocytes demonstrable by conventional staining. These cells in flow microfluorometry studies expressed antigens Leu 4 (T‐cell antigen receptor), Leu 7 (natural killer cell marker), Leu 2 (suppressor cell marker), and HLA‐DR (activation marker); they lacked Leu 1 (a pan‐T cell antigen), Leu 3 (helper cell marker) and Tac (interleukin 2 receptor). Hematopoietic colony formation in vitro improved with addition of corticosteroids to the culture medium or elimination of the LGL population with complement‐mediated cytotoxicity. Anti‐neutrophil antibodies were present prior to and following therapy. Clinically, administration of prednisone resulted in a normalization of the total white blood cell count and absolute polymorphonuclear cell number, an increase into the normal range of the number of CFU‐C, and elimination of the LGL population. In this case of steroid‐responsive LGL‐associated neutropenia, laboratory studies suggested direct suppression of myelopoiesis by stero
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