Fibrinopeptide A (FPA), a sensitive index of in vivo thrombin activity, β-thromboglobulin (βTG) and platelet factor 4 (PF4), specific markers of platelet intravascular activation, have been measured in plasma by radioimmunoassays in 23 patients with nephrotic syndrome and in 32 normal subjects. FPA concentration was 2.40 ± 1.42 ng/ml (mean ± SD) in nephrotic patients and 1.16 ± 0.58 ng/ml in normal controls (p < 0.001); βTG concentration was 57.9 ± 33.2 ng/ml in nephrotic patients and 25.7 ± 7.4 ng/ml in controls (p < 0.001); PF4 level was not different from controls. These data indicate in vivo blood hypercoagulability and platelet hyperfunction in nephrotic syndrome. Moreover, we have documented a slow in vitro FPA generation pattern (ΔFPA): 0.97 ± 0.51 ng/ml/l0 min; this suggests that thrombin activity is predominan