Introduction. The significance of ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin motif-13) activity for diagnosis and therapy of thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) is still a controversial issue. Objective. The aim of this report was to analyze the value of ADAMTS13 measurements in the diagnosis of TTP and HUS. Methods. At presentation, we analyzed patients with idiopathic TTP (n=18), secondary TTP (n=4), diarrhea positive HUS (n=3) and diarrhea negative HUS (n=3) treated in Belgrade, Serbia from 2004 to 2010. ADAMTS13 activity from acute phase samples was measured using the residual collagen binding activity assay at the Haemophilia and Thrombosis Centre, Milan, Italy. Results. There was a significant correlation between reduced ADAMTS13 activity and idiopathic TTP diagnosis (p=0.000) as well as between lower ADAMTS13 activities and higher reticulocytes (p=0.017) and lactate dehydrogenase levels (p=0.027). Significant correlation was also found between higher protease activity and diagnosis of HUS (p=0.000). There was a statistically significant correlation between higher ADAMTS13 activities and higher platelets count (p=0.002), blood urea nitrogen (p=0.000), and creatinine level (p=0.000). Conclusion. Severe ADAMTS13 deficiency points at the diagnosis of idiopathic TTP and it is present in the secondary TTP but not in HUS.