Myelokathexis is a congenital disorder that causes severe chronic leukopenia and neutropenia. Characteristic findings include degenerative changes and hypersegmentation of mature neutrophils and hyperplasia of bone marrow myeloid cells. The associated neutropenia can be partially corrected by treatment with granulocyte colony-stimulating factor (G-CSF) or granulocyte–macrophage colony-stimulating factor (GM-CSF). These features led us to propose that accelerated apoptosis of neutrophil precursors might account for the neutropenic phenotype. Blood and bone marrow aspirates were obtained from 4 patients (2 unrelated families) with myelokathexis before G-CSF therapy and from 2 of the affected persons after G-CSF therapy (1 μg/kg per day subcutaneously for 3 weeks). Bone marrow was fractionated using immunomagnetic bead cell sorting into CD34⁺, CD33⁺/CD34⁻, and CD15⁺/CD34⁻/CD33⁻ cell populations. Examination of these cells by flow cytometry and electron microscopy revealed abundant apoptosis in the CD15⁺ neutrophil precursor population, characterized by enhanced annexin-V binding, extensive membrane blebbing, condensation of heterochromatin, and cell fragmentation. Colony-forming assays demonstrated significant reduction in a proportion of bone marrow myeloid-committed progenitor cells. Immunohistochemical analysis revealed a selective decrease inbcl-x, but not bcl-2, expression in the CD15⁺/CD34⁻/CD33⁻ cell population compared with similar subpopulations of control bone marrow-derived myeloid precursors. After G-CSF therapy, apoptotic features of patients' bone marrow cells were substantially reduced, and the absolute neutrophil counts (ANC) and expression ofbcl-x in CD15⁺/CD34⁻/CD33⁻cells increased. The authors concluded that myelokathexis is a disease characterized by the accelerated apoptosis of granulocytes and the depressed expression of bcl-x in bone marrow-derived granulocyte precursor cells. These abnormalities are partially corrected by the in vivo administration of G-CSF. (Blood. 2000;95:320-327)