During their invasion of the placental bed, interstitial trophoblasts fuse to multinuclear giant cells which are thought to have lost their invasive properties. Trophoblast fusion is associated with downregulation of E-cadherin, and persistent E-cadherin expression has been linked to defective placentation in preeclampsia. Since a previous study suggested 'premature' giant cell formation in preeclampsia, we started with the working hypotheses that fusion is increased in hypertensive pregnancies, and that the intensity of fusion correlates with the severity of disease. Using double immunostaining for E-cadherin and cytokeratin 7/17, nuclei in interstitially invasive trophoblasts (IT) in the myometrial compartment of the placental bed from normotensive pregnancies (NT, n=8), gestational hypertension (GH, n=4), preeclampsia (PE, n=9), and HELLP syndrome (n=5) were categorised according to the E-cadherin staining of the cell and their occurrence in single, clustered or multinuclear cells. GH and PE patients showed a higher percentage of nuclei in clustered non-fused E-cadherin-positive cells (P<0.01 and P<0.05), and in smaller (bi- and trinuclear) placental bed giant cells (P<0.05) compared to NT pregnancies, suggesting defective IT fusion. In contrast, in HELLP syndrome no such failed fusion could be discerned, which may support the idea of a heterogeneous aetiology of different hypertensive diseases of pregnancy. Since we are still ignorant about the specific role of mononuclear and multinuclear trophoblasts in the placental bed, it is not yet possible to relate the present findings to the pathogenesis of different categories of hypertensive pregnancies.