Overexpression of the epidermal growth factor (EGF) receptor occurs frequently in ovarian cancer and is associated with poor patient prognosis. A constitutively active mutant EGF receptor termed variant III (EGFRvIII) has been detected at a high frequency in many human tumors, including those of the ovary. To identify the consequences of EGFRvIII expression in ovarian tumor cells, we introduced EGFRvIII into the epithelial ovarian cancer cell line (OVCA 433). The EGFRvIII-transfected cells displayed a dissociated, motile phenotype and fibroblastic morphology. The EGFRvIII-dependent phenotype was comparable to that observed in EGF-stimulated parental OVCA 433 cultures and required the catalytic activity of the mutant receptor. Disruption of adherens and desmosomal junctions in EGFRvIII expressing cells was evident by immunofluorescent detection of specific junctional components. In addition, Western blot analysis confirmed decreased levels of cellular plakoglobin and beta-catenin in EGFRvIII-expressing cells, and E-cadherin protein and mRNA were nearly absent. The loss of E-cadherin was accompanied by decreased expression of additional ovarian epithelial markers, including keratins 7, 8, and 18 and mucins 1 and 4. In contrast, the mesenchymal markers N-cadherin and vimentin were elevated in EGFRvIII expressing cells. Overall, the switch in cadherins from E-cadherin to N-cadherin, coupled with gain of vimentin expression and loss of the epithelial keratins and mucins typically expressed in well-differentiated epithelial ovarian carcinomas, are consistent with transition to a mesenchymal phenotype as an outcome of EGFRvIII expression. These findings suggest that EGFRvIII expression may regulate phenotypic plasticity in ovarian cancer and thereby contribute to more aggressive disease.