SummaryIn order to compare the plasmatic progressive antithrombin activity to the concentration of three thrombin inhibitors, antithrombin III (AT III), α2 macroglobulin (α2, M), α1 anti-trypsin (α1 AT) in nephrotic syndrome, a prospective study was carried out on a group of 28 children affected with the disease. A dramatic reduction of the level of AT III and of α1 AT, two inhibitors of molecular weight close to that of albumin, was observed. The decreased level of AT III was counterbalanced by an increase in α2 M. This phenomenon accounts for the increased progressive antithrombin activity observed in all the affected children. It is suggested that the above compensatory mechanism explains the absence of thrombotic accidents in this series and that the benefit of heparin therapy is doubtful in these conditions.