Eosinophilia has long been associated with endomyocardial fibrosis, but the involvement of the eosinophilia in fibrosis of other organs is unclear. To investigate this question, the authors tested whether tissue eosinophilia and eosinophil degranulation are present in syndromes associated with fibrosis. The authors used an indirect immunofluorescent technique to localize eosinophil granule major basic protein (MBP) in formalin-fixed, paraffin-embedded tissue specimens from 50 patients. Thirty-four specimens were obtained from patients with inflammatory fibrosis: 12 with idiopathic retroperitoneal fibrosis, seven with sclerosing mediastinitis, four with sclerosing cholangitis, and 11 with pulmonary fibrosis. The remaining 16 specimens were obtained from patients with noninflammatory fibrous proliferations: four with keloids, six with scars, three with Dupuytren's contracture and three with dense stromal fibrosis of the breast. Eosinophil infiltration and/or extracellular MBP deposition were observed in 28 of the 34 specimens (82%) from patients with inflammatory fibrosis, including 11 of the 12 cases of retroperitoneal fibrosis, five of the seven cases of sclerosing mediastinitis, all four cases of sclerosing cholangitis, and 8 of the 11 cases of pulmonary fibrosis. In contrast, eosinophil infiltration and MBP deposition were not observed in specimens from the 16 patients with noninflammatory fibrous proliferation (P less than 0.001). These results indicate that eosinophil infiltration and release of a granule protein, namely MBP, commonly occur in inflammatory fibrotic lesions.