To address the complex chronic effector properties of interleukin (IL)-10, we generated transgenic mice in which IL-10 was overexpressed in the lung. In these mice, IL-10 inhibited endotoxin-induced tumor necrosis factor production and neutrophil accumulation. IL-10 also caused mucus metaplasia, B and T cell-rich inflammation, and subepithelial fibrosis and augmented the levels of mRNA encoding Gob-5, mucins, and IL-13. In mice bred to have null mutations of IL-13, IL-4R(alpha), or STAT-6, transgenic IL-10 did not induce mucus metaplasia but did induce inflammation and fibrosis. IL-10 was also a critical mucin regulator of virus-induced mucus metaplasia. Thus, IL-10, although inhibiting lipopolysaccharide-induced inflammation, also causes mucus metaplasia, tissue inflammation, and airway fibrosis. These responses are mediated by multiple mechanisms with mucus metaplasia being dependent on and the inflammation and fibrosis being independent of an IL-13/IL-4R(alpha)/STAT-6 activation pathway.