Objective: To examine the prevalence of isolated IgA anti- β2-glycoprotein I (anti-β2GPI) positivity and the association of these antibodies, and a subgroup that bind specifically to domain IV/V of β2GPI, with clinical manifestations of the antiphospholipid syndrome (APS) in 3 patient groups and to evaluate the pathogenicity of IgA anti-β2GPI in a mouse model of thrombosis. Methods: Patients with systemic lupus erythematosus (SLE) from a multiethnic, multicenter cohort (LUpus in MInorities, NAture versus nurture [LUMINA]) (n = 558), patients with SLE from the Hopkins Lupus Cohort (n = 215), and serum samples referred to the Antiphospholipid Standardization Laboratory (APLS) (n = 5,098) were evaluated. IgA anti-β2GPI titers and binding to domain IV/V of β2GPI were examined by enzyme-linked immunosorbent assay (ELISA). CD1 mice were inoculated with purified IgA anti-β2GPI antibodies, and surgical procedures and ELISAs were performed to evaluate thrombus development and tissue factor (TF) activity. Results: A total of 198 patients were found to be positive for IgA anti-β2GPI isotype, and 57 patients were positive exclusively for IgA anti-β2GPI antibodies. Of these, 13 of 23 patients (56.5%) in the LUMINA cohort, 17 of 17 patients (100%) in the Hopkins cohort, and 10 of 17 patients (58.9%) referred to APLS had at least one APS-related clinical manifestation. Fifty-four percent of all the IgA anti- β2GPI-positive serum samples reacted with domain IV/V of anti-β2GPI, and 77% of those had clinical features of APS. Isolated IgA anti-β2GPI positivity was associated with an increased risk of arterial thrombosis (P < 0.001), venous thrombosis (P = 0.015), and all thrombosis (P < 0.001). The association between isolated IgA anti-β2GPI and arterial thrombosis (P = 0.0003) and all thrombosis (P = 0.0003) remained significant after adjusting for other risk factors for thrombosis. In vivo mouse studies demonstrated that IgA anti-β2GPI antibodies induced significantly larger thrombi and higher TF levels compared to controls. Conclusion: Isolated IgA anti-β2GPI-positive titers may identify additional patients with clinical features of APS. Testing for these antibodies when other antiphospholipid tests are negative and APS is suspected is recommended. IgA anti-β2GPI antibodies directed to domain IV/V of β2GPI represent an important subgroup of clinically relevant antiphospholipids. © 2013 by the American College of Rheumatology.