T ransforming growth factor beta (TGF beta) is a multifunctional protein which has been suggested to play a central role in the pathogenesis of chronic inflammation and fibrosis. Nasal polyposis is a condition affecting the upper airways characterized by the presence of chronic inflammation and varying degrees of fibrosis. To examine the potential role of TGF beta in the pathogenesis of this condition, we investigated gene expression and cytokine production in nasal polyp tissues as well as in the normal nasal mucosa. By Northern blot analysis using a porcine TGF beta 1 cDNA probe, we detected TGF beta 1-specific mRNA in nasal polyp tissues, as well as in the tissue from a patient with allergic rhinitis, but not in the normal nasal mucosa. By the combination of tissue section staining with chromotrope 2R with in situ hybridization using the same TGF beta 1 probe, we found that approximately 50% of the eosinophils infiltrating the polyp tissue express the TGF beta 1 gene. In addition, immunohistochemical localization of TGF beta 1 was detected associated with extracellular matrix as well as in cells in the stroma. These results suggest that in nasal polyposis where eosinophils are the most prevalent inflammatory cell, TGF beta 1 synthesized by these cells may contribute to the structural abnormalities such as stromal fibrosis and basement membrane thickening which characterize this disease.