We sought insights into the classification of and factors associated with relapse severity and disease stability in neuromyelitis optica spectrum disorder (NMOSD) clinical practice worldwide.Neurologists recruited from six countries (the USA, Germany, Italy, Brazil, South Korea, and China) participated in a 30-60 minute online survey and submitted two to four clinical records for aquaporin-4-immunoglobulin G (AQP4-IgG)-seropositive adults with NMOSD, which included patient demographics, diagnosis, maintenance treatment history, relapse occurrence, and severity. Separately, patients with NMOSD receiving maintenance therapy were interviewed over the telephone about their treatment journey, as well as perceptions of relapse severity and disease stability, and their potential influence on treatment decisions.Clinical records for 1185 patients with AQP4-IgG-seropositive NMOSD were provided by 389 neurologists (July-August 2020); 33 patients were interviewed (October-November 2020). There was no clear consensus on how relapse severity was defined in clinical practice, with geographical variations in relapse classification also found. Neurologists tended to rely on clinical assessments when determining severity, viewing each relapse in isolation, whereas patients had a more subjective view based on the changes in their daily lives and comparisons with prior relapses. Similarly, there was a disconnect in the definition of disease stability: the complete absence of relapses was more important for patients than for neurologists.A clear consensus on how to assess relapse severity and disease stability is needed to ensure that patients receive appropriate and timely treatment. In the future, clinical measures should be combined with patient-focused assessments.© 2023. The Author(s).