To evaluate the role of p27Kip1 in tumorigenesis and the development of trophoblastic cell disease.Using immunohistochemistry, the expression of p27-protein was investigated in 10 normal chorionic villi in the first trimester of pregnancy, 15 complete hydatidiform moles (HM), 7 invasive moles (IM) and 7 choriocarcinomas (CC).In all cases, immunohistochemical staining localized p27-protein in the plasma. Decreased expression of p27Kip1 was observed in malignant trophoblastic neoplasms with a positive rate of 21.43%, which is significantly less than that in normal chorionic villi (80%) and in complete HM (73.33%) (P < 0.05). The positive rate of p27Kip1 in those complete HM with large uterine size for gestational age was lower than that in those with normal or small uterus (42.86% vs 100%, P < 0.05).p27Kip1 may be involved in the tumorigenesis of gestational trophoblastic neoplasm as a negative regulator of the cell cycle. The expression level of p27Kip1 in trophoblastic cells may be a prognostic factor for complete HM.