Soluble fibrin (SF) is produced by activated blood coagulation reaction and is useful to diagnose thrombotic diseases. We measured plasma and urine SF levels in nephritic patients to assess the hypercoagulability state associated with the disease. Before they received anti-coagulation or anti-platelet therapies, 60 patients underwent measurement of plasma SF and D-dimer levels by Latex agglutination turbidimetric immnoassay (LA). Urinary SF levels were also measured by LA. Plasma and urinary thrombin antithrombin III complex (TAT) levels were measured by enzyme immunoassay (EIA). Plasma SF levels showed a good correlation with plasma TAT levels but only weak positive correlations were observed between plasma D-dimer and SF or TAT levels. Plasma SF and D-dimer levels were significantly higher in the Iatients with nephrotic-range hypoalbuminemia (< or =3 g/dL) than those without it. Contrarily there was no significant difference in plasma TAT levels between these two groups of patients. In almost all patients, urinary SF levels were under the detection limit. However, TAT was excreted into urine more frequently in patients showing the nephrotic range of hypoalbuminemia at 38.2% than in non-nephrotic patients at 8.0%. Thus, plasma SF levels more precisely indicate activated blood coagulation reaction than plasma TAT levels in nephrotic patients, probably because the plasma SF is not excreted into urine, while plasma TAT is.