The published data concerning changes of antithrombin III (ATIII) in nephrotic syndrome (NS) are contradictory. While increased ATIII activity has been reported by some investigators, decreased concentration has been shown by others and normal values by yet another group of authors. We determined plasma and urine concentrations of ATIII in a group of 20 patients with NS using an immunologic assay. In addition, plasma ATIII activity was determined. The results were compared with those obtained in a group of normal volunteers. Plasma concentration and activity of ATIII were both greatly reduced in the patients with NS. In addition, substantial quantities of ATIII were recovered in the urine of all tested patients. The present study, therefore, substantiates the low plasma concentrations of ATIII and its urinary losses in NS. In addition, a parallel reduction in plasma ATIII activity is demonstrated providing functional evidence of acquired ATIII deficiency in this condition.