AM J RESPIR CRIT CARE MED 1999;160:S49−S52.An imbalance between neutrophil proteases and the surrounding antiproteases is critical in the normal functioning of the neutrophil. Enzyme activity is of importance in cell migration and may play a role in some beneficial aspects of host defense. However, when persistent or excessive this imbalance can be detrimental and (even in the absence of antiprotease deficiency) central to most of the pathogenic processes in COPD. Understanding of these complex relationships that alter a beneficial host defense response to a destructive one will be critical in the development of long-term therapeutic strategies. Stockley RA. Neutrophils and protease/antiprotease imbalance.