Asthma is classically defined in terms of reversible airflow obstruction. It is now recognized that histologic abnormalities, including inflammation, are a regular feature of asthma. In addition, alterations of the structural elements of the airway wall occur regularly in asthma. These alterations include a thickened basal lamina comprised of interstitial collagens as well as alterations in the mesenchymal cell population with an accumulation of myofibroblasts. It seems likely that these connective tissue alterations in the airway wall contribute to the physiologic abnormalities of asthma. While controversial, the long-term physiologic sequela of asthma may depend in large part on these changes. The biochemical and cellular basis that leads to these changes is, as yet, unknown, but recent studies suggest that a variety of cells in the airway, including inflammatory cells and the cells of the airway epithelium, may participate in regulating this response.