The role for matrix metalloproteinases (MMPs) in tumor cells invasion and metastasis is well established, and expression of MMPs is recognized as an indication of tumor cell malignancy. Previous studies suggest that the degradation of the basement membrane is a crucial early step in epithelial transformation and ovarian tumorigenesis. Thus, MMPs may also express and exert a role in preneoplastic lesions of ovarian tissues. We investigated the expression of the major metalloproteinases, gelatinase A, 72 kDa type IV collagenase (MMP-2), and gelatinase B, 92 kDa type IV collagenase (MMP-9), and the presence of basement membrane in ovarian tumors and tissues from prophylactic oophorectomies using immunostaining. MMP expression was also characterized in a panel of ovarian cancer cell lines and several nontumorigenic ovarian surface epithelial primary cells by zymography, Northern, and Western blots. We found, surprisingly, that MMP-2 and MMP-9 are expressed more frequently in early lesions than in established carcinomas. No correlation was found between the expression of MMPs and tumor grades or stages. In preneoplastic lesions, MMP-2 or MMP-9 expression often associates with the absence of basement membrane and morphological alterations. MMP-2 is often expressed in nontumorigenic ovarian surface epithelial cells but reduced or absent in cancer cells. Thus, we conclude that MMPs expression does not correlate with the malignancy of ovarian epithelial cells as generally thought. Rather, increased metalloproteinase expression is an early event in ovarian tumorigenesis and associates with the loss of epithelial basement membrane and morphological transformation. We propose that the increased MMP activity is an etiological factor for ovarian cancer risk. We found that MMPs expression does not correlate with the malignancy of ovarian epithelial cells as generally thought. Rather, increased metalloproteinase expression is an early event in ovarian tumorigenesis. The finding suggests roles of MMP in tumor initiation in addition to invasion, and may impact on the strategy for use of MMP inhibitors in cancer prevention. © 2006 Wiley-Liss, Inc.