The cyclin-dependent kinase inhibitors (CDKIs) inhibit the kinase activities of the cyclin and CDK complexes and block transitions of the cell cycle. As a negative regulator of the cell cycle, CDKI P27 kip1 regulates the decision of cell cycle to enter into S-phase or arrested in G1-phase by blocking G1 cyclin dependent kinase activities. Evidence suggests that P27 kip1 may be involved in tumorigenesis and acts as a tumor suppressor gene. In this study, the expression of P27 kip1 in complete hydatidiform mole (CHM), invasive mole (IM) and choriocarcinoma (CC) was investigated as well as in normal chorionic villi by immunohistochemistry. Decreased expression of P27 kip1 was observed in malignant trophoblastic neoplasms with a positive rate of 21.43%, which is less than that in normal chorionic villi (80%) and in CHM (73.33%) (P<0.05). The positive rate of P27 kip1 in those CHMs with large uterus size for gestational date was lower than that in those with a normal or small uterus for date (42.86% vs 100%). (P<0.05). These findings suggest that P27 kip1 may be involved in the tumorigenesis of gestational trophoblastic neoplasm as a negative regulator of cell cycle, and the expression level of P27 kip1 in trophoblastic cells may be a prognostic factor for CHM.