A new family of cytokines, IL-17, has recently been defined that reveals a distinct ligand-receptor signaling system. Functional studies have provided evidence for its importance in the regulation of immune responses. Notably, 3 members, IL-17A, IL-17E (IL-25), and IL-17F, have been best characterized both in vitro and in vivo , and have been shown to be proinflammatory in nature. This proinflammatory activity is exemplified by their involvement in pulmonary inflammatory responses, in which both IL-17A and IL-17F are involved in the recruitment of neutrophils, and IL-17E is able to induce T H 2 cytokine production and eosinophilia. Although the elucidation of a detailed mechanism of action continues to be an active area of research, the potent inflammatory activity and its association with various human disease states suggest this new cytokine family as an important contributor to the pathophysiology of human disease conditions, in particular the pulmonary diseases.