While the neoplastic cells have always been in the center of interest in cancer research in recent years, more and more attention has been paid also to the tumor stroma which is today known to play an important role for tumor progression. In early days of histology the interpretation of stroma as either neoplastic or reactive was still a matter of discussion. Today's generally held view is clearly in favor of a secondary induction by tumor cell derived factors. Recent findings of epithelium-mesenchymal transitions of cultured tumor cells now put again a doubt on this exclusive interpretation. In the present work we show for the first time that genetic alterations (allelic losses, microsatellite instabilities and p53 mutations) are not restricted to tumor cells but also occur within the fibroblastic stroma of non-hereditary invasive human colon and breast cancers. In view of the clonality of epithelial and stromal components in the same cases (loss of same alleles and identical MSI patterns) epithelium-mesenchymal transitions are among possible explanations for these findings which could shed a new light on the true nature of tumor stroma