c-Myc oncoprotein is overexpressed in a significant proportion of human epithelial cancers, and experimental overexpression of c-Myc in epithelial cells promotes tumour formation. However, it is not known how c-Myc promotes epithelial cell tumour formation. We report that c-Myc expression in human mammary epithelial cells induces a dramatic change in cell morphology, with some characteristics of an ‘epithelial to mesenchymal transition’. E-cadherin expression is repressed by a post-transcriptional mechanism in cells expressing c-Myc. Furthermore, E-cadherin repression is necessary for c-Myc-induced cell transformation.