Airway smooth muscle (ASM), an important tissue involved in the regulation of bronchomotor tone, exists in the trachea and in the bronchial tree up to the terminal bronchioles. The physiological relevance of ASM in healthy airways remains unclear. Evidence, however, suggests that ASM undergoes marked phenotypic modulation in lung development and in disease states such as asthma, chronic bronchitis and emphysema. The shortening of ASM regulates airway luminal diameter and modulates airway resistance, which can be augmented by cytokines as well as extracellular matrix alterations. ASM may also serve immunomodulatory functions, which are mediated by the secretion of pro-inflammatory mediators such as cytokines and chemokines. In addition, ASM mass increases in chronic airway diseases and may represent either a pathologic or an injury-repair response due to chronic inflammation. This review will present evidence that ASM, a "passive" contractile tissue, may become an "active participant" in modulating inflammation in chronic lung diseases. Cell facts 1. Found in the trachea and along the bronchial tree. 2. Critically important in regulating bronchomotor tone of the airways. 3. Differentiation state is associated with the expression of various "contractile proteins." 4. Displays phenotypic modulation of mechanical, synthetic and proliferative responses. 5. Secretes cytokines, chemokines and extracellular matrix proteins. 6. May serve as a potential new target for the treatment of chronic lung diseases.