Neutrophils have a crucial function in the defense against bacteria and fungi. Indeed, during chronic, severe neutropenia and in case of severe neutrophil dysfunctions, the patients may suffer recurrent and sometimes life-threatening infections. This article describes the clinical symptoms, the theory behind the antimicrobial systems of neutrophils, the methods to diagnose the various aberrations, and the possibilities for treating these patients. A few of the most common causes of neutropenia and neutrophil dysfunctions are described in detail, including recent genetic information regarding the cause of these diseases. (C) 1999 Elsevier Science B.V. All rights reserved.