Differential diagnosis between complete and partial hydatidiform mole is clinically important because of the difference in the risk of developing more malignant form of the molar diseases. In this report, the classical microscopic criteria were re-evaluated in the light of the immunohistochemistry of p57KIP2 in the attempt to establish robust morphological criteria for the differential diagnosis. Thirty-six consecutive cases clinically suspected to be hydatidiform mole were employed. The histological criteria were scored by three pathologists. The cases were categorized into three entities of the molar diseases in accordance with the immunohistochemistry of p57KIP2 and CD34. The diagnostic significance of the histological criteria was evaluated in a logistic regression model. Of 36 cases, the immunohistochemistry revealed that 28 were complete and 6 were partial hydatidiform mole, while 2 cases were hydropic abortion. A stepwise logistic regression analysis indicated that, among seven criteria studied, three of them (shape of villi, prevalence of villi with three types of trophoblasts, and predominance of villi with hydropic change) were useful to differentiate complete hydatidiform mole from partial one. This observation may be applicable in the pathological diagnosis of the molar diseases.