Severe forms of idiopathic interstitial pneumonia (IIP), such as usual interstitial pneumonia (UIP), can be impervious to modern steroid and immunosuppressive treatment regimens, thereby emphasizing the need for novel effective therapies. Understanding the cytokine networks that may affect immune and structural cell activation and, hence, the progression of these fatal fibrotic diseases, has been a focus in our research. In this regard, we have examined the role of interleukin (IL)-4 and IL-13 and their respective receptor subunits in this process. Examination of clinical surgical lung biopsies (SLBs) showed that IIP is characterized by the abnormal, heightened expression of the receptor subunits that bind IL-4 and IL-13. Specifically, IL-4Ralpha and IL-13Ralpha2 (the high-affinity IL-13 receptor subunit) was present in greater abundance in SLBs and fibroblasts from IIP patients compared with normal patients, who exhibited no evidence of pulmonary fibrosis. These clinical findings prompted us to investigate whether the targeting of pulmonary cell types that were highly responsive to IL-4 and IL-13 was a viable therapeutic option in IIP. Using a chimeric protein comprised of human IL-13 and a truncated version of an exotoxin from Pseudomonas (abbreviated IL13-PE), we observed that IL13-PE selectively targeted human pulmonary fibroblasts grown from IIP SLBs, whereas it had a minimal effect on fibroblasts grown from biopsies from normal patients. In murine models characterized by abnormal airway or interstitial fibrotic responses, the intranasal administration of IL13-PE significantly attenuated the fibrotic response through the targeting of IL-4Ralpha- and IL-13Ralpha2-expressing pulmonary cells, including monocytes, macrophages, and pulmonary fibroblasts. Together, these data demonstrate that IL-4 and IL-13 are required for the initiation and maintenance of pulmonary fibrosis, and highlight the importance of further investigation of anti-fibrotic therapeutics that prevent the action of both cytokines during clinical pulmonary fibrosis.