The clinical significance of ADAMTS13 activity for response to treatment, mortality rate, recurrence, and prognosis is unclear. Therefore, we investigated the characteristics of severe ADAMTS13 deficiency and evaluated its prognostic features in Thrombotic thrombocytopenic purpura (TTP). The Korean TTP Registry includes 66 patients from 13 teaching hospitals in Korea who received the diagnosis of TTP from January 2005 to December 2008. Blood samples obtained upon admission were sent for ADAMTS13 analysis (multimer analysis by sodium dodecyl sulfate electrophoresis) to a central laboratory along with patient clinical information. Patients with severe ADAMTS13 deficiency had lower serum creatinine levels (P = 0.001) than patients with non-severe ADAMTS13 deficiency. Although severe ADAMTS13 deficiency was associated with better response rate (75 vs. 53%, P = 0.145), remission rate (81 vs. 61%, P = 0.209), and mortality rate (19 vs. 31%, P = 0.508) than non-severe ADAMTS13 deficiency, treatment outcomes did not differ significantly between groups. After adjusting for clinical and laboratory features, multivariate analysis did not reveal any independent risk factors for TTP-associated mortality. Patients with severe ADAMTS13 deficiency had lower serum creatinine levels at presentation, but severe ADAMTS13 activity deficiency at TTP diagnosis does not appear to have prognostic significance.