Autoantibodies neutralizing human ADAMTS13 (a disintegrin-like and metalloproteinase with thrombospondin type 1 motif), the metalloprotease that physiologically cleaves von Willebrand factor, are a major cause of severe deficiency of the protease and of acquired thrombotic thrombocytopenic purpura (TTP). We evaluated prevalence of anti-ADAMTS13 antibodies in 59 patients with thrombotic microangiopathies (TMAs) and in 160 patients with immunologic or thrombocytopenic diseases different from TTP, using an enzyme-linked immunosorbent assay (ELISA). Immunoglobulin G (IgG) antibodies directed against ADAMTS13 were found in 97% of untreated patients with acute acquired TMA who had plasma levels of ADAMTS13 activity below 10%. The corresponding prevalence of IgM antibodies was 11%. In contrast, anti-ADAMTS13 antibodies of G or M isotypes were detected in 20% of patients with TMA with ADAMTS13 activity above 10%. The ELISA was more sensitive than the standard functional inhibitor assay for detecting antibodies against ADAMTS13. Patients with thrombocytopenia from various causes (n = 50), systemic lupus erythematosus (SLE; n = 40), and the antiphospholipid antibody syndrome (APS; n = 55) had prevalences of IgG antibodies of 8%, 13%, and 5% respectively, only slightly higher than the prevalence in 111 healthy donors (4%). A rather high prevalence of anti-ADAMTS13 IgM antibodies was found in patients with SLE and APS (18% each). The clinical significance of IgM antibodies in these groups is unclear. In conclusion, the ELISA method detected anti-ADAMTS13 IgG antibodies in a very large proportion of patients with acquired TMA associated with severe ADAMTS13 deficiency, and was more sensitive than the inhibitor assay.