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Antiphospholipid (aPL) antibodies have historically been postulated to cause a poorly understood inflammatory myelitis. Neuromyelitis optica spectrum disorder (NMOSD) causes an inflammatory longitudinally extensive transverse myelitis (LETM). In 2004, aquaporin-4 immunoglobulin G (AQP4-IgG) was first reported as a highly specific (>99%) serum diagnostic biomarker of NMOSD, distinguishing it from other disorders (eg, multiple sclerosis). We sought to assess the frequency of AQP4-IgG (and thus NMOSD diagnosis) in LETM with aPL antibodies. We searched Mayo Clinic records (from January 1, 1996, through December 31, 2014) for patients with (1) LETM and (2) aPL or β2-glycoprotein I antibodies and (3) a serum sample available. AQP4-IgG was evaluated in the 24 included patients and in 20 controls with aPL antibodies but without myelitis. Seropositivity for AQP4-IgG was confirmed in 11 of 24 patients with LETM (46%), confirming an AQP4-IgG–seropositive NMOSD diagnosis rather than aPL-associated LETM. Six of 11 AQP4-IgG–seropositive patients (54%) were initially diagnosed as having aPL/lupus-associated myelitis. Recurrent LETM was exclusive to AQP4-IgG–seropositive patients (P=.003). Alternative diagnoses assigned to the remaining 13 AQP4-IgG–seronegative patients included idiopathic transverse myelitis (n=5), seronegative NMOSD (n=2), spinal cord infarct attributed to aPL antibodies (n=2), spinal cord sarcoidosis (n=1), varicella-zoster virus myelitis (n=1), postinfectious myelitis (n=1), and multiple sclerosis (n=1). All 20 controls were seronegative for AQP4-IgG. Clotting disorders occurred in 36% of patients (4 of 11) with LETM with both aPL antibodies and AQP4-IgG. AQP4-IgG should be tested in all patients with LETM and aPL antibodies because AQP4-IgG–seropositive NMOSD accounts for almost half of all cases. Clotting disorders are common in patients with LETM with dual positivity for AQP4-IgG and aPL antibodies.