Plasma levels of D-dimer, a degradation product of crosslinked fibrin, were determined in 73 patients with breast cancer. In these patients, significantly elevated plasma levels of D-dimer were found, as compared to healthy controls (P less than 0.0001). In addition, we observed a moderate correlation between plasma levels of D-dimer and those of CA15-3 (r = 0.40; P less than 0.001) and between D-dimer and carcinoembryonic antigen (r = 0.39; P less than 0.01). Plasma levels of thrombin-antithrombin III complex (TAT), reflecting the activation of thrombin, were also significantly elevated in patients with breast cancer (P less than 0.0001), and a poor, but significant correlation between carcinoembryonic antigen and TAT (r = 0.25; P less than 0.05) was found. We concluded that the increase in plasma D-dimer and TAT levels might reflect an enhanced activation of the clotting system in patients with breast cancer.