This study examines the proliferative activity of trophoblast in hydatidiform mole, non-molar hydropic abortion and non-molar spontaneous abortion. Nine cases of complete mole, 10 cases of partial mole, eight cases of non-molar hydropic abortion and six cases of non-hydropic second trimester abortion were examined by routine histopathology and the rate of cell proliferation was assessed by immunoreactivity for proliferating cell nuclear antigen (PCNA). Hydropic abortion showed a significantly lower PCNA index than complete mole and partial mole. There was no significant difference in PCNA index between partial mole and non-hydropic abortion. The trophoblast of partial hydatidiform mole demonstrates significant cell proliferation but this, although higher than that of hydropic abortion, is no higher than that of non-hydropic abortion of a similar gestational age. The role of partial mole as a precursor of persistent gestational trophoblastis disease remains unclear.