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International classification criteria for antiphospholipid syndrome (APS) include IgM (immunoglobulin M), aCL (anticardiolipin), and aB2GPI (anti-β2-glycoprotein-I) antibodies, but their relevance is still debated. We aimed to assess whether patients with isolated IgM aCL and/or aB2GPI at diagnosis have specific characteristics and outcomes.We retrospectively included APS patients with isolated IgM antiphospholipid antibodies (isolated-IgM-APS) and compared them to APS patients with IgG and IgM, or IgG alone and/or lupus anticoagulant (nonisolated-IgM-APS).Among the 168 APS patients included, 24 (14.3%) had isolated IgM. Median follow-up was 92.5 months (36–151.5). Isolated-IgM-APS patients were 9.5 years older. At diagnosis, stroke was more frequent in isolated-IgM-APS after adjustment for cardiovascular risk factors (odds ratio, 3.8; 95% CI, 1.3–11.5). IgM isotype remained isolated in 17 of 24 (70.8%) patients over time. Global relapse-free survival did not differ between the two groups. In thrombotic APS, monotherapy with antiplatelet agents was more frequently used in isolated-IgM-APS group with 14 of 20 versus 28 of 134 patients (P<0.0001), with a higher relapse rate with antiplatelet agent alone compared to vitamin K antagonists, especially for patients presenting with a stroke (hazard ratio, 7.37; 95% CI, 1.19–19.0).Isolated IgM APS patients should not be disregarded because they represent 14.3% of an APS population. They have some characteristics: older age at diagnosis and a strong association with stroke. Clinicians must be aware of this situation because antiplatelet agent do not seem to well prevent relapses compared to vitamin K antagonist.