The proteinase-antiproteinase hypothesis still receives support from clinical and experimental observations in a range of inflammatory lung diseases. The function of these molecules appears to be broader than originally believed and further research is likely to lead to an improved understanding of their role in the regulation of both the beneficial and detrimental effects in inflammatory response and the maintenance of the homeostasis in the normal lung. Thus the potential for the development as therapeutic tools is likely to become more attractive as improved drug development and delivery mechanisms appear.