In the Nephrotic Syndrome an hypercoagulable state can cause an increased incidence of thromboembolic phenomena and the course of the syndrome. The deficiency of Antithrombin III has been suggested to explain the hypercoagulability. We measured plasma antithrombin concentration (as percentage) in 24 children suffering from N.S. and the values were correlated with serum albumin, proteinuria and Partial Thromboplastin Time (PTT). The results of this study show that plasma Antithrombin III (AT-III) is significantly correlated with serum albumin, with proteinuria and PTT. Moreover plasma AT-III concentration was found to be low particularly when patients relapsed and in 2 children who developed thombophlebitis of the safena vein.