Transforming growth factor-β1 (TGF-β1) is a multipotent growth factor affecting development, homeostasis and tissue repair. Many kinds of malignant tissues were reported to overexpress transforming growth factor-β1 (TGF-β1) gene. However, a little work has been done on the circulating TGF-β1 and the association of TGF-β1 with progression in patients with malignant tumors. In this study, we measured the plasma level of TGF-β1 in gastric cancer and prostate cancer patients and evaluated the utility of plasma TGF-β1 as a possible tumor marker. We used Enzyme-linked immunosorbent assay (ELISA) system in order to measure plasma TGF-β1 level in 134 gastric cancer patients, 50 prostate cancer patients and 290 normal controls. And the tumor marker, carcinoembryonic antigen (CEA), prostate-specific antigen (PSA), was compared with TGF-β1 in the aspects of sensitivity and specificity. The mean plasma TGF-β1 levels were 1.219±0.834 (0.272-5.772) ng/mL in normal controls, 5.964±3.218 (0.845-18.124) ng/mL in gastric cancer and 4.140±2.345 (1.108-13.302) ng/mL in prostate cancer. In gastric cancer patients difference in plasma TGF-β1 level was not detected according to cancer stage. In comparison with other tumor marker (CEA, PSA) TGF-β1 is more potent in sensitivity. These results indicate that the plasma TGF-β1 level can be a potent tumor marker in gastric cancer and prostate cancer.