Fibroblastic foci, a major histological feature of usual interstitial pneumonia (UIP), play a critical role in the development of UIP. The mechanisms involved in the formation of these foci, however, including cellular origin, remain unclear. Recent in vitro and animal studies suggested epithelial-mesenchymal transition (EMT) of alveolar epithelial cells during pulmonary fibrogenesis. The aim of the present study was to investigate the presence of EMT in patients with UIP on quantitative immunohistochemistry using pathological tissue sections. The study subjects were 13 patients with UIP pattern among 52 patients with interstitial pneumonia who underwent lung biopsy. Alveolar epithelial cells overlying fibroblastic foci expressed epithelial markers less frequently and mesenchymal markers more frequently compared with those in non-diseased control lung tissues (n= 10). Moreover, double immunostaining showed that some epithelial cells stained for both epithelial and mesenchymal markers. Furthermore, significantly higher numbers of epithelial marker-positive fibroblastic cells were found in fibroblastic foci in UIP as well as in other non-UIP fibrosing diseases than in control lung tissues. The results showed that some epithelial cells overlying fibroblastic foci lose the epithelial phenotype and gain the mesenchymal phenotype, and that some fibroblastic cells in fibroblastic foci originate from epithelial cells. But this EMT may not be specific for UIP.