The expression of oestrogen and progestogen receptors, as well as Cathepsin D, an oestrogen-induced protease, in trophoblastic cells of 12 cases of complete hydatidiform mole, 12 cases of partial mole and nine cases of spontaneous abortions, were studied in an attempt to elucidate their possible roles in the pathogenesis of the diseases. The immunohistochemical studies were performed on formalin-fixed paraffin-embedded tissue using the ABC immunoperoxidase method. The intensity of staining and proportion of cells stained were assessed and compared in the three categories of lesions. Immunoreactivity for Cathepsin D was noted in both the syncytio- and cytotrophoblastic cells in all three lesions. Statistical analysis showed consistently stronger and more extensive staining for Cathepsin Din complete moles when compared with abortions. Staining for oestrogen and progestogen receptors was found to be weak in the tissues studied. The strong expression of Cathepsin D in trophoblastic cells of all three lesions, especially in complete mole, suggests that it might be important in the control of trophoblastic cell activities and involved in the pathogenesis of hydatidiform mole. The associated weak expression of sex hormone receptors also suggests that the expression of Cathepsin D in trophoblastic cells may be controlled by modes of regulation other than sex hormones.