Abstract: Objective To study the expression of cathepsins B and L in first-trimester gestational decidua and chorionic villi. Methods The decidua and chorionic villi in the first trimester of gestation were obtained from 30 women undergoing induced abortion, 25 with spontaneous abortion, 10 with normal endometrium in secretory phase, and 15 with bydatidiform mole in whom the expression of cathepsins B and L was determined by immunohistochemistry. Results Positive staining for cathepsins B and L were mainly detected in the trophoblasts and decidual cells. The positive expression rate of cathepsin B in the normal endometrium of secretory phase, decidua in the first trimester of gestation and spontaneous abortion were 10% (1/10), 83.3% (25/30), and 32.0% (8/25), respectively, and the rate of cathepsin L expression in the endometrium in secretory phase, decidua of induced abortion and spontaneous abortion were 0.0%, 63.3% (19/30), and 32.0% (8/25) respectively, showing significant difference in the expression rates of cathepsins B and L between the 3 groups (P<0.05). Strong positive expression of cathepsin B in chorionic villi of induced abortion, spontaneous abortion and hydatidiform mole were detected at the rates of 10.0% (3/30), 0.0%, and 66.7% (10/15), respectively, and cathepsin L at the rates of 26.7% (8/30), 4.0% (1/25), and 80.0% (12/15), respectively, with significant difference in strong cathepsins B and L expressions between the 3 groups (P<0.05). Conclusion The expression of cathepsins B and L differs in normal and abnormal deciduas and chorionic villi of early pregnancy, suggesting that the cathepsins may play important roles in the process of implantation.