Kostmann's disease (KD) is a severe congenital neutropenia, a rare autosomal recessive disorder of neutrophil number. Complete blood count with differential reveals an ANC less than 500/mm3 as seen in classic cases, monocytosis and eosinophilia. Bone marrow aspiration or biopsy reveals an arrest of neutrophil precursor maturation at the promielocyte or myelocyte level. KD results in an increased susceptibility to frequent bacterial infections. In the absence of medical intervention with granulocyte colony-stimulating factor (G-CSF), bone marrow transplantation, the mortality rate is 70% within the first year of life. We present three cases to illustrate this rare entity, the difficulty of diagnosis, but also to underline the importance of correct treatment.