Flow cytometry represents an interesting methodologic approach to human neutrophil biology and pathology. Several aspects of neutrophil activation can be evaluated by flow cytometry: phagocytosis, respiratory burst (superoxide anion generation, intracellular hydrogen peroxide production), intracellular pH, actin polymerization, membrane potential, aggregation, cytotoxicity, degranulation, and surface marker expression. In several instances whole blood methods have been developed and have been shown to be very specific and sensitive. Neutrophils can easily be distinguished from other circulating cells, including eosinophils, and methods for the evaluation of neutrophil phagocytosis and hydrogen peroxide production are currently used. In addition whole blood procedures for the determination of the expression of neutrophil function-associated surface antigens have been recommended in order to avoid the artifactual changes resulting from isolation methods. Purified neutrophil have to be used to study other parameters such as actin polymerization and cytotoxicity. Flow cytometry permits a multiparametric evaluation of neutrophil functions, and advances in software technology facilitate a wide variety of choices for the handling, storage, and evaluation of data. The applications of flow cytometry in human neutrophil diseases include the diagnosis of chronic granulomatous disease and leukocyte adhesion syndrome types 1 and 2, and the evaluation of granulocyte biology in several clinical conditions such as immunodeficiency conditions, recurrent infections, hematologic diseases, and noninfectious diseases caused by uncontrolled neutrophil activation.