The pathogenesis of endometriosis is unknown, but retrograde menstruation is widely accepted as an etiology. Refluxed endometrium from endometriosis patients is more prone to implant and invade peritoneum possibly through the action of extracellular proteolysis. This proteolytic action may involve plasminogen activators and the collagenase system. Plasminogen activators (PAs) and matrix metalloproteinases (MMPs) play a critical role in the breakdown of extracellular matrix components and basement membrane in the processes of implantation and tumor invasion. PAs are inhibited by plasminogen activator inhibitor (PAI) and MMPs activity is inhibited by tissue inhibitor of metalloproteinase (TIMP). To test the hypothesis that lower expression of PAI-1 and TIMP-3 in endometrium from women with endometriosis, we investigated their PAI-1 and TIMP-3 expression by quantitative competitive RT PCR in endometrium from women with and without endometriosis. Endometrial tissues were obtained from 14 patients with severe endometriosis and 14 patients without endometriosis. Total RNA was extracted and reverse transcribed into cDNA, and quantitative competitive PCR (QC PCR) was performed to evaluate PAI-1 and TIMP-3 mRNA expression. Endometrium from patients with endometriosis showed decreased expression of PAI-1 and TIMP-3 mRNA compared to endometrium from control in luteal phase (p<0.05). Our results suggest that endometrium from women with endometriosis expresses lower levels of PAI-1 and TIMP-3 than endometrium from normal women. Endometrium from endometriosis patients may be more invasive and prone to peritoneal implantation than control because of higher PA and MMP enzymatic activity. Thus, increased proteolytic activity may be one of the reasons for the invasive properties of the endometrium resulting in the development of endometriosis.