Plasma protein C (PC) activity, prothrombin time (PT), partial thromboplastin time (PTT) and platelet count were studied in 23 children with nephrotic syndrome (NS) and AT III activity was determined in 15 children in the same group. All parameters were compared with those obtained in a group of 16 age matched healthy controls. Mean plasma AT III activity was significantly reduced in patients with NS (P < 0.001) correspondingly, plasma AT III levels were found to be directly correlated with serum albumin and inversely correlated with proteinuria. In contrast, mean plasma PC activity, as well as PT, PTT and platelet count were similar in the NS group when compared with the control group and in addition no remarkable difference was found in the mean plasma PC activity between the steroid sensitive and resistant NS groups. In conclusion, this study demonstrated acquired AT III deficiency and normal PC levels in childhood NS. These data suggest that although plasma AT III activity depends on the severity of NS, neither the severity of NS nor the underlying renal disease is an important factor determining the changes of PC activity in childhood NS.