Background: The Morquio A Registry Study (MARS) is an ongoing, multinational, observational study of patients with MPS IVA. Key objectives of MARS are to characterize the heterogeneity and natural history of disease and to evaluate long-term effectiveness and safety of elosulfase alfa enzyme replacement therapy (ERT). Enrollment began in September 2014; data on medical history, clinical outcomes, and safety assessments are collected as part of routine care., Results: As of February 2021, 381 subjects from 17 countries had enrolled in MARS: 58 ERT-naïve subjects and 323 ERT-treated subjects (≥1 infusion), with a mean ERT exposure of 5.5 years (SD 2.8) and median age at first ERT treatment of 9.8 years. ERT-treated subjects were younger at diagnosis (median 3.4 vs 6.5 years) relative to ERT-naïve subjects. Among ERT-treated subjects, urinary keratan sulfate (uKS) levels declined from pre-ERT baseline to last follow-up on treatment (mean % change [95% confidence interval]: -52.5% [-57.5%, -47.4%]; n = 115) and 6-min walk test distance remained stable (mean change: -6.1 [-27.6, 15.5] m; n = 131) over a mean follow-up of 5.5 years. Forced expiratory volume in 1 s (FEV 1 ) and forced vital capacity (FVC) increased in subjects who were < 18 years of age at ERT initiation (mean change: +0.3 [0.1, 0.4] L and + 0.4 [0.3, 0.5] L; mean follow-up: ∼6 years; n = 82) and were stable in subjects ≥18 years (mean change: 0.0 [-0.0, 0.1] L and 0.0 [-0.1, 0.1] L; mean follow-up: 4.6 years; n = 38). Overall, 148 (47.1%) ERT-treated subjects experienced ≥1 adverse event (AE) and 110 subjects (35%) reported ≥1 serious AE. Drug-related AEs were reported in 39 (12.4%) subjects; the most common were hypersensitivity (9 subjects [2.9%]), urticaria (8 subjects [2.5%]), and pyrexia (7 subjects [2.2%])., Conclusions: MARS is the longest and largest observational study of MPS IVA patients to date, with a heterogenous population that is representative of the MPS IVA population overall. Data collected over the first 6 years of MARS provide real-world evidence for long-term stabilization of endurance and respiratory function among ERT-treated patients, with no new safety concerns identified., Competing Interests: Declaration of Competing Interest JM reports advisory boards for BioMarin, Genzyme, Takeda, and Ultragenyx, consulting fees from BioMarin, Genzyme, Takeda, and Ultragenyx, contracted research for BioMarin, Genzyme, RegenxBio, and Takeda, honoraria from BioMarin and Genzyme and speaker's bureau for BioMarin. BKB reports consulting fees from Aeglea, BioMarin, Denali, Horizon, JCR Pharma, Moderna, Shire (Takeda), SIO, and Ultragenyx, contracted research for BioMarin, Denali, Homology Medicines, Sangamo, Shire (Takeda), and Ultragenyx, and speaker's bureau for BioMarin, Horizon, and Shire (Takeda). MBB reports advisory boards, consulting fees, and contracted research for BioMarin, and contracted research for RegenxBio. PMC reports consulting fees from BioMarin and Genzyme, and contracted research for BioMarin, Genzyme, and Takeda. CE reports advisory boards for Amicus, BioMarin, Chiesi, and Sanofi Genzyme, consulting fees from BioMarin and Sanofi Genzyme, honoraria from BioMarin, Sanofi Genzyme, and Takeda Shire, speaker's bureau for Amicus, BioMarin, Sanofi Genzyme, and Takeda Shire, and travel expenses from Amicus, BioMarin, Sanofi Genzyme, and Takeda Shire. KB has no conflicts of interest. NG reports advisory boards for BioMarin, Chiesi, Sanofi Genzyme, Takeda Shire HGT, and Ultragenyx, consulting fees from Sanofi Genzyme, and Ultragenyx, contracted research for BioMarin, Chiesi, Sanofi Genzyme, Shire HGT, and Ultragenyx, and travel expenses from Sanofi Genzyme, and Ultragenyx. SL reports advisory boards for BioMarin. MM reports advisory board for BioMarin and speaker's honoraria from BioMarin, Chiesi, and Takeda. JR reports advisory boards for BioMarin, Sanofi, and Takeda. Honoraria from BioMarin, Sanofi, and Takeda, and travel expenses from BioMarin, Sanofi, and Takeda. KMS reports advisory boards for BioMarin, Chiesi, Orchard Therapeutics, Sanofi, and Takeda, and honoraria from BioMarin, Orchard Therapeutics, Sanofi, and Takeda, SC, SD, DH, and AL are employees of BioMarin. LS-S was an employee of BioMarin at the time of submission of the manuscript., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)