Abstract: Objective To investigate the role of matrix metalloproteinase-9 (MMP-9) and tumor necrosis factor-α (TNF-α) in the pathogenesis of pregnancy-induced hypertension (PIH). Methods The placenta samples were collected from 57 patients with PIH and 24 normal pregnant women. Immunohisochemical staining was used to examine positive expression of MMP-9 and TNF-α in these samples. Results The positive expression of MMP-9 in the normal group was significantly higher than that in patients with moderate or severe PIH (P<0.01), while no significant difference was found between normal pregnant women and patients with mild PIH (P>0.05), and expression decreased drastically with the progression of PIH (P<0.05). The expression of TNF-α in the patients with mild, moderate and severe PIH groups were significantly higher than that in the normal group (P<0.05, P<0.01 and P<0.01, respectively), and the differences were also significant between the PIH groups of different severities (P<0.01). No correlation was found between MMP-9 and TNF-α expressions in the normal and mild PIH groups (r=0.287, P>0.05; r=0.382, P>0.05), in the patients with moderate and severe PIH, MMP-9 expression showed inverse correlation with TNF-α expression (r=-0.563, P<0.05; r=-0.681, P<0.01). Conclusion Loweed MMP-9 expression in syncytio- trophoblast might result in defective nidation in the placenta where local ischemia and hypoxia cause abnormal TNF-α elevation, which might be one of the important factors inducing PIH.