I n normal human pregnancy, invasion of the uterus and its arterial system by cytotrophoblasts extends through the entire decidua and the adjacent third of the myometrium. Our previous work showed that during the first trimester of pregnancy, invasion is accompanied by a marked change in the expression of cell adhesion molecules by invasive cytotrophoblasts. In the pregnancy disorder preeclampsia, cytotrophoblast invasion is limited to the superficial decidua, and few arterioles are breached. The purpose of this study was to determine whether cytotrophoblast expression of adhesion molecules in this disorder is also abnormal. Placental bed biopsy specimens from normal pregnancies and those complicated by preeclampsia were stained with anti-integrin antibodies. The results showed that adhesion molecule switching by invasive cytotrophoblasts is abnormal in preeclampsia, which suggests that this subpopulation of trophoblast cells fails to differentiate properly. A likely result is that the delicate balance of adhesive interactions that normally permit cytotrophoblast invasion is tipped in favor of those which restrain this process, with the net effect of shallow uterine invasion.