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We have demonstrated that fibroblasts associated with carcinomas stimulate tumor progression of initiated non-tumorigenic epithelial cells both in an in vivo tissue recombination system and in an in vitro co-culture system. Human carcinoma-associated fibroblasts grown with initiated human epithelial cells dramatically stimulate growth and alter histology of epithelial cells. This effect is not detected when normal fibroblasts are grown with the same epithelial cells under the same experimental conditions. From these data and other data we conclude that, in this human cancer model, carcinoma-associated fibroblasts stimulate tumor progression of an initiated epithelial cell.