Fibrin-related markers (FRMs), such as fibrin and fibrinogen degradation products (FDPs), D-dimer, and soluble fibrin (SF), are considered to be useful for the diagnosis of thrombosis. However, the evidence for the making of a diagnosis of thrombosis based on FRMs is, as yet, not fully established. Levels of FRMs are significantly elevated in patients with thrombosis, such as deep vein thrombosis, disseminated intravascular coagulation, and so on. In Japan, the D-dimer assay test result might be 2-fold higher than results for those assays commonly used in Europe and North America. The levels of SF are significantly elevated in patients before the onset of thrombosis, thus suggesting that the SF assay is useful not only for the diagnosis of thrombosis but also for diagnosing a prethrombotic state. Overall, elevated levels of FRMs indicate a high risk for thrombosis, and they are thus considered to be useful for the diagnosis of thrombosis.