Quantitative changes of hemostatic molecular markers were studied in patients with nephrotic syndrome. The plasma levels of fibrinopeptide A (FPA), thrombin-antithrombin III complex (TAT), products of thrombin activation, and fragment F1 + 2 (F1 + 2), a product of prothrombin activation, were measured by enzyme immunoassay in 21 patients with nephrotic syndrome and in 16 normal controls. The mean value of FPA was 17.5 +/- 7.5 ng/ml (mean +/- SD) in nephrotic patients and 4.5 +/- 0.3 ng/ml in normal controls (P < 0.02); F1 + 2 concentration was 1.4 +/- 0.3 ng/ml in nephrotic patients and 0.5 +/- 0.1 ng/ml in normal controls (P < 0.001); TAT level was 1.0 +/- 0.3 microgram/l in nephrotic patients and 0.2 +/- 0.1 microgram/l in normal controls (P < 0.05). These data indicated intravascular hemostasis activation. Based on these results, we propose that low antithrombin III level in nephrotic patients may be due to both urinary loss and intravascular consumption.