OBJECTIVES: Baseline data from the Morquio A Clinical Assessment Program (MorCAP) revealed that individuals with Morquio A syndrome show substantial impairment in multiple domains including endurance and respiratory function (Harmatz et al., Mol Genet Metab, 2013). Here, 1- and 2-year longitudinal endurance and respiratory function data are presented. METHODS: Endurance was assessed using the 6-minute walk test (6MWT) and the 3-minute stair climb test (3MSCT). Respiratory function was evaluated by measuring forced vital capacity (FVC) and maximum voluntary ventilation (MVV). Data were analyzed using repeated measures ANCOVA models. Annualized estimates of change were determined using model estimates and interpolation. RESULTS: 353, 184, and 78 subjects were assessed at Year 0 (baseline), Year 1, and Year 2, respectively. The overall annualized estimate of change (SE) in 6MWT distance was -4.86+/-3.25m; a larger decline of -6.84+/-5.38m was observed in the subset of subjects meeting the inclusion/exclusion criteria of the Phase 3 clinical trial of elosulfase alfa (>/=5years of age with baseline 6MWT distance >/=30 and =325m). In contrast, little change (-0.14+/-0.60stairs/min) was observed in 3MSCT. Annualized changes (SE) in FVC and MVV were 2.44+/-0.68% and 1.01+/-2.38%, respectively. FVC and MVV increased in patients aged =14years, but decreased in older patients. CONCLUSIONS: The natural history of Morquio A syndrome is characterized by progressive impairment of endurance as measured by the 6MWT. Longitudinal trends in FVC and MVV showing increase in younger patients, but decrease in older patients, are likely to be influenced by growth. Changes in 6MWT may represent a sensitive measure of disease progression in ambulatory Morquio A patients.