Complete hydatidiform moles (CHM) and partial hydatidiform moles (PHM) represent different clinicopathologic entities. To obtain prognostic and therapeutic information about both entities, it is important that pathologic classification be as accurate as possible. The distinction of molar pregnancy and an abortus with hydropic changes (AHC) can sometimes be very difficult. The acquisition of 2 antibodies against nuclear antigens expressed in cycling cells, Ki-67 and proliferating cell nuclear antigen (PCNA), allow the study of trophoblastic proliferation in CHM, PHM, and AHC. The purpose of this study is to determine whether immunocytochemical stains can help in the distinction between those entities. All materials were obtained by curettage from 95 patients with hydropic villi evident on microscopic examination. The 95 cases included 33 cases of CHM, 42 cases of PHM, and 20 cases of AHC. In the case of the Ki-67 staining, the mean was much lower in the ACH group (8.7%) than in the PHM group (65.3%) or in the CHM group (84.6%). In the case of PCNA staining, the mean differences among the 3 groups (AHC, 23.1%; PHM, 80%; and CHM, 89.2%) were all statistically significant. On the basis of the means and the Gaussian results, it appears that the Ki-67 distribution gives a better separation among the 3 groups. In conclusion, proliferative activity is an additional useful parameter for evaluation of molar pregnancies and hydropic changes, with Ki-67 staining allowing better separation than PCNA staining does.