The hemolytic-uremic syndrome is a predominantly pediatriccondition that consists of the simultaneous triad of acute renal failure,thrombocytopenia, and microangiopathic hemolytic anemia.ADAMTS13(a disintegrin and metalloprotease with thrombospondin motifs), acirculating multidomain zinc metalloprotease of the reprolysin subfamily,is critical for preventing von Willebrand factor-platelet interaction underhigh shear stress conditions.Considering the overlapping clinicalmanifestation of Hemolytic uremic syndrome (HUS) and Thromboticthrombocytopenic purpura (TTP), we hypothesized that ADAMTS13would be an important factor in discrimination between the two TMAs.This is a case control study carried out on children hospitalized in thePediatric Hospital, Cairo University during the one-year period fromDecember 2011 to December 2012.This study aimed at understanding therole of ADAMTS13 enzyme in the pathogenesis of hemolytic uremicsyndrome (HUS) patients and the correlation of its activity with differentpresenting symptoms.The study included 25 HUS patients, 60% malesand 40% females below the age of 18 years. ADAMTS13 enzymeactivity was measured using ELISA technique, results ranged betweennormal and mild to moderate deficiency. None of our cases showedmarked deficiency. ADAMTS13 Inhibitor was also measured in deficientcases which showed negative results. The measurement of ADAMTS13activity and anti-ADAMTS13 antibodies at the time of diagnosis couldaid in the discrimination between TTP and HUS and hence treatmentdecision.