Antithrombin III (AT II/III) was determined immunologically and by means of a heparin cofactor assay in plasma samples and 24-hour urine of 15 patients with various degrees of proteinuria, being predominantly of glomerular origin. In urine the AT II/III concentrations were significantly correlated to the concentrations of albumin, plasminogen and IgG. One third of the patients had AT II/III plasma levels below the normal range. The plasma levels showed a significant inverse correlation to the AT II/III and albumin clearance rates. Similarily, the plasminogen concentrations in plasma were decreased in two thirds of the patients, being inversely correlated to the renal plasminogen clearance values. It is proposed that AT II/III deficiency in the nephrotic syndrome is an important pathogenetic factor in venous thrombosis.