Fibrin-related markers, such as fibrin and fibrinogen degradation products (FDP) and D-dimer, are considered useful for the diagnosis of thrombosis. However, the evidence for making a diagnosis of thrombosis based on fibrinrelated markers is still not yet well established. The plasma concentrations of soluble fibrin and D-dimer were prospectively measured in 680 inpatients suspected of having thrombosis between October 1, 2003 and January 31, 2005, and correlated with thrombosis. The normal ranges of D-dimer and FDP were within 0.76 μg/ml and 1.50 μg/ml, respectively. Out of 680 patients, 129 patients showed plasma concentrations associated with thrombosis, including 73 with deep venous thrombosis (DVT)/ pulmonary embolism (PE). The plasma D-dimer and FDP concentrations were significantly higher in the patients with thrombosis than in the patients without thrombosis, but there were no significant differences in the D-dimer and FDP levels among the patients with thrombosis. The plasma D-dimer levels were significantly correlated with the plasma FDP levels in all the patients and there was no significant difference in the ratio of FDP/ D-dimer among the various diseases. A ROC analysis showed that both FDP and D-dimer were useful for the diagnosis of all types of thrombosis and DVT. The cutoff values of D-dimer (3.8 μg/ml) and FDP (7.7 μg/ml) had high sensitivity, specificity and negative predictive values (NPV) but low positive predictive value. Our findings suggest that FDP showed a close correlation with D-dimer, which is known to be a marker for a hypercoagulable state, and it is also reflects a high risk for thrombosis.