We investigated zeb1, twist and claudins 1 and 4 in normal and diseased placental tissues. Eighty cases of placental tissues from all three trimesters and from different diseases were studied immunohistochemically to determine the expression of zeb1, twist and claudins 1 and 4. Zeb1 was more strongly expressed in endothelial and mesenchymal cells of the villous structures during the last two trimesters, suggesting that it plays a role in the development of placental blood vessels. In contrast, twist was more strongly expressed in the trophoblastic cells during these trimesters. The endothelial and stromal expression of zeb1 and the epithelial expression of twist were disturbed in those placentas with chromosomal aberrations. Claudin 4 was strongly expressed in trophoblastic cells during all trimesters. Its expression was especially strong in molar disease, suggesting that it could participate in trophoblastic aggregation and disturbed attachment of trophoblastic cells in the expanded villi. Claudin 4 expression was also increased in the placentas of diabetic mothers and in toxaemia. Claudin 1 did not show any specific disease associations. The study implicates that twist and zeb1 are involved in placental maturation, whereas claudin 4 appears to be connected with placental diseases such as diabetes, toxaemia or molar disease.