Summary. Background: ADAMTS13-neutralizing IgG autoantibodies are the major cause of acquired thrombotic thrombocytopenic purpura (TTP). Objective: To analyze the IgG subclass distribution of anti-ADAMTS13 antibodies and a potential relationship between subclass distribution and disease prognosis. Methodology: An enzyme-linked immunosorbent assay-based method was used to quantify the relative amounts of IgG subclasses of anti-ADAMTS13 antibodies in acquired TTP plasma. Results: IgG4 (52/58, 90%) was the most prevalent IgG subclass in patients with acquired TTP, followed by IgG1 (52%), IgG2 (50%), and IgG3 (33%). IgG4 was found either alone (17/52) or with other IgG subclasses (35/52). IgG4 was not detected in 10% of the patients. There was an inverse correlation between the frequency and abundance of IgG4 and IgG1 antibodies (P < 0.01). Patients with high IgG4 levels and undetectable IgG1 are more prone to relapse than patients with low IgG4 levels and detectable IgG1. Conclusions: All IgG subclasses of anti-ADAMTS13 antibodies were detected in patients with acquired TTP, with IgG4, followed by IgG1, antibodies dominating the anti-ADAMTS13 immune response. Levels of IgG4 could be useful for the identification of patients at risk of disease recurrence.