We analyzed blood plasma concentrations of matrix metalloproteinase-1 and -3 (MMP-1; MMP-3), the tissue inhibitor of metalloproteinase-1 (TIMP-1) and the complex MMP-1/ TIMP-1, and looked for any correlation with prostate cancer stage. These components were measured by ELISA tests specific for these proteins in healthy male controls (n 5 35), and in patients with benign prostatic hyperplasia (BPH; n 5 29), with prostate cancer (PCa) without metastasis (T2,3pN0M0; n 5 29) and with PCa with metastatic disease (T2,3,4pN1,2M1; n 5 18). Mean values of MMP-1 and of the complex MMP-1/TIMP-1 were not different among the 4 groups studied. The mean MMP-3 and especially TIMP-1 concentrations were significantly higher in PCa patients with metastases compared with controls, BPH and PCa patients without metastases. Ten of these 18 patients had TIMP-1 concentrations higher than the upper reference limit. TIMP-1 concentrations were correlated with staging but not with grading. Our results point towards plasma TIMP-1 concentration as a potential marker of malignant progression of PCa. Matrix metalloproteinases (MMP) are a family of enzymes with the common ability to degrade various components of the extracellular matrix such as collagen, elastin and gelatin (Liotta and Stetler-Stevenson, 1991). These enzymes are involved in all physiological processes occurring during tissue remodelling and repair and play a crucial role in pathological conditions such as rheumatoid arthritis, tumor invasion and metastasis (Crawford and Matrisian, 1995). Both in vitro and in vivo investigations have shown that increased concentrations of MMPs are associated with the invasive and metastatic potential in several human malignant tumors, e.g., in breast, colon, gastric and lung cancers (Anderson et al., 1995;Zucker et al., 1995;Murray et al., 1996). It is assumed that these proteases destroy the integrity of the basement membrane and enable cancer cells to invade normal tissue and facilitate tumor cell dissemination via blood vessels and lymphatics. The catalytic activities of MMPs are controlled by various mechanisms including enzyme synthesis, secretion as zymogens which undergo extracellular activation and inhibition of the activated enzymes by specific inhibitors, the tissue inhibitors of metalloproteinases (TIMPs). Thus, the balance between MMPs and TIMPs as both positive and negative modulators of the invasive and metastatic processes appears to be decisive (Crawford and Matrisian, 1995).Since these changes in cellular concentration may be reflected in body fluids, determinations of MMPs, TIMPs and of their complexes in the blood have been recommended as simple, noninvasive tools in cancer diagnostics and monitoring (Zucker et al., 1995). Increased plasma concentrations of the complex gelatinase B:tissue inhibitor have been observed in patients with gastrointestinal tract cancer (Zucker et al., 1995). There has been only one report on MMPs in the blood of prostate cancer (PCa) patients: Baker et al. (1994) noted changes in the concentration of...