Chronic mucosal airway inflammation is currently considered to be the central element in the pathophysiology of asthma. This inflammatory process is regulated by a complex network of mutually interacting cytokines. The overall pattern of cytokine expression in asthma is one of increased expression of T-helper 2-like cytokines, including interleukin (IL)-4 and IL-5, in addition to proinflammatory cytokines, such as tumour necrosis factor-alpha (TNF-alpha) and IL-1 beta. The precise functional importance of each of these cytokines within the global network remains to be fully established. This question has been addressed in a number of in vivo animal studies. Combining these data with descriptive findings in man makes it possible to address somewhat further the question of the in vivo significance of a specific cytokine in the asthmatic state of the airways. This paper reviews some of the data currently available on this subject.