AbstractCongenital dysgranulopietic neutropenia (CDN) is a recently proposed entity that describes a small subgroup of children with clinically severe neutropenia. We followed and studied a 3‐year‐old girl with neutropenia (<500/mm3) and recurrent severe infections in whom repeated marrow evaluations revealed large (30‐50 μm) multinucleated promyelocytes to polymorphonuclear cells with as many as 4 to 16 nuclei or nuclear lobes, respectively. In addition to the nuclear endoreduplication, ultrastructural and cytochemical evaluation of these cells demonstrated abnormalities in granule genesis and centriole structure. Concomitantly. immunoperoxidase staining indicated that many of the granules were devoid of lactoferrin but not lysozyme. In vitro proliferation studies revealed normal to increased thymidine labeling, normal numbers of colony‐forming cells, and normal colony‐stimulating activity from blood and marrow mononuclear cells, findings consistent with ineffective myelopoiesis. However, serum folate, B12, and lysozyme levels were normal. The nuclear and cytoplasmic abnormalities in this patient result in an extreme example of CDN, distinct from previously descr