Because endometriotic tissue is capable of implantation and invasion, it resembles neoplastic tissue. This biologic behavior is likely regulated by cell adhesion molecules and enzymes that degrade the extracellular matrix. A potential factor in the local regulation of tumor cell invasion is the urokinase-type plasminogen activator (uPA) system. This study was undertaken to characterize the expression of both uPA and its receptor (uPAR) in the eutopic and heterotopic endometrial cells from women with adenomyosis by testing both types of epithelial cells for invasive ability. By Western blot and zymographic analyses, stromal cells from heterotopic endometrium secreted 12 times more uPA antigen and uPA-dependent caseinolytic activity than did the eutopic stromal cells. The heterotopic epithelial cells overexpressed uPAR by a factor of four times the expression seen in the eutopic epithelial cells. When respective stromal cells were cocultured, the heterotopic epithelial cells exhibited significantly higher invasive ability through Matrigel than did the eutopic epithelial cells. uPAR overexpression in the epithelial cells and high secretion of uPA from the stromal cells may contribute to the invasive phenotype of heterotopic endometrium.