The receptor tyrosine kinase RON (recepteur d'origine nantais), a member of the MET proto-oncogene family, has been implicated in the pathogenesis of certain epithelial cancers including lung adenocarcinomas. To determine the oncogenic potential of RON, transgenic mice were generated using the surfactant protein C promoter to express human wild-type RON in the distal lung epithelial cells. The mice were born normal without morphological defects in the lung, however, multiple lung adenomas with distinct morphology and growth pattern were observed. Tumors appeared as a single mass in the lung around 2 months of age and gradually developed into multiple nodules throughout the lung. Most of the tumors were characterized as cuboidal epithelial cells with type Ⅱ cell phenotypes. They grew along the alveolar walls and projected into the alveolar septa. A transition from premalignant adenomas to adenocarcinomas was observed. The RON transgene is highly expressed and constitutively activated in the tumors as evident by immunohistochemical staining and western blot analyses. Moreover, we found that Ras expression was dramatically increased in the majority of tumors. However, no mutation in the 'hot spots' of the K-Ras or p53 gene was observed, although limited genomic instability occurs in individual tumors. Taken together, this is a mouse lung tumor model with unique biological characteristics. The model may provide an opportunity to study the role of RON in lung tumors and to elucidate the mechanisms underlying this distinct lung tumor.