The present study investigated the intracellular regulation of E-cadherin in ovarian carcinoma. E-cadherin expression and regulation by Snail and Pak1 were studied in ES-2 and OVCAR-3 ovarian cancer cells in vitro. Twist1, Zeb1 and Vimentin mRNA expression and HIF-1alpha protein expression were analyzed in 80 and 189 clinical specimens, respectively. OVCAR-3 cells incubated with an anti-E-cadherin antibody formed smaller and looser spheroids compared to controls. Snail silencing using Small Hairpin RNA in ES-2 cells reduced invasion and MMP-2 activity, with unaltered cellular morphology. Using dominant negative (DN) and constitutively active (CA) Pak1 constructs, we found that DN Pak1 ES-2 and OVCAR-3 clones had reduced attachment to matrix proteins, invasion and MMP-2 activity compared to CA and wild-type cells. DN Pak1 ES-2 cells also bound less to LP9 mesothelial cells. DN Pak1 OVCAR-3 cells had lower Vimentin levels. Snail expression was lower in cultured effusions compared to primary carcinomas, and was cytoplasmic rather than nuclear. Twist1 (P < 0.001), Zeb1 (P = 0.003) and Vimentin (P = 0.03) mRNA expression was significantly higher in solid metastases compared to primary carcinomas and effusions. HIF-1alpha protein expression was lower in effusions compared to primary carcinomas and solid metastases (P = 0.033). Our data suggest that the previously reported E-cadherin re-expression in ovarian carcinoma effusions is regulated by Pak1. The transient nature of E-cadherin expression during ovarian carcinoma progression is probably the result of partial epithelial-to-mesenchymal transition (EMT) and the reverse process of mesenchymal-to-epithelial-like transition (MET). Expression of the EMT-related molecules Twist, Zeb1, Vimentin and HIF-1alpha is anatomic site-dependent in ovarian carcinoma.