STAT3 is a latent transcription factor that plays a role in regulating fibroblast function in fibrotic lung diseases. To further understand the role of STAT3 in the phenotypic divergence and function of human lung fibroblasts (LFs), we investigated the effect of basal and cytokine-induced STAT3 activity on indices of LF differentiation and activation, including expression of alpha-smooth muscle actin (alpha-SMA), collagen, and adhesion molecules Thy-1/CD90 and alpha(v) beta(3) and beta(5) integrins. We identified a population of fibroblasts from usual interstitial pneumonia (UIP)/idiopathic pulmonary fibrosis (IPF) lungs characterized by constitutively phosphorylated STAT3, lower proliferation rates, and diminished expression of alpha-SMA, Thy-1/CD90, and beta(3) integrins compared with control LFs. Staining of UIP lung biopsy specimens demonstrated that phosphorylated STAT3 was not present in alpha-SMA-positive fibroblastic foci but was observed in the nuclei of cells located in the areas of dense fibrosis. STAT3 activation in LFs did not significantly influence basal or transforming growth factor beta(1)-induced collagen I expression but inhibited expression of alpha-SMA, Thy-1/CD90, and alphav beta(3) integrins. Suppression of STAT3 signaling diminished resistance of IPF LFs to staurosporine-induced apoptosis and responsiveness to transforming growth factor beta(1) but increased basal alpha-SMA and restored beta(3) integrin expression in LFs via an ALK-5-dependent, SMAD3/7-independent mechanism. These data suggest that STAT3 activation regulates several pathways in human LFs associated with normal wound healing, whereas aberrant STAT3 signaling plays a critical role in UIP/IPF pathogenesis.