•Natural history changes in pulmonary function tests across time in Duchenne muscular dystrophy.•Treatment with glucocorticoids (steroids) > 1 year was compared to steroid naïve treatment.•Steroid treatment slowed the rate of pulmonary decline as measured by FVC%p in 7–9.9 year olds;•Steroid treatment resulted in higher peak absolute FVC and PEFr values with later onset of decline.•Time to FVC < 1 liter was delayed by steroid treatment; FVC < 1 liter increased risk of death 4-fold. We describe changes in pulmonary function measures across time in Duchenne muscular dystrophy patients treated with glucocorticoids (GCs) > 1 year compared to GC naïve patients in the Cooperative International Research Group Duchenne Natural History Study, a multicenter prospective cohort study. 397 participants underwent 2799 pulmonary function assessments over a period up to 10 years. Fifty-three GC naïve participants (< 1 month exposure) were compared to 322 subjects with > 1 year cumulative GC treatment. Forced vital capacity (FVC), peak expiratory flow rate (PEFr), maximal inspiratory and expiratory pressures were performed and calculated as a percent predicted (%p). GC treatment slowed the rate of pulmonary decline as measured by FVC%p, in patients aged 7–9.9 years. GC treatment slowed 12 and 24-month progression of percent predicted spirometry to a greater degree in those with baseline FVC%p from < 80–34%. GC treatment resulted in higher peak absolute FVC and PEFr values with later onset of decline. Progression to an absolute FVC < 1 liter was delayed by GC treatment. Patients who reached a FVC below 1 L were 4.1 times more likely to die (p = 0.017). Long-term glucocorticoid treatment slows pulmonary disease progression in Duchenne dystrophy throughout the lifespan.