BACKGROUND: Determine the effects of a 12-week respiratory muscle training (RMT) program in late-onset Pompe disease (LOPD). METHODS: We investigated the effects of 12-weeks of RMT followed by 3-months detraining using a single-subject A-B-A experimental design replicated across 8 adults with LOPD. To assess maximal volitional respiratory strength, our primary outcomes were maximum inspiratory pressure (MIP) and maximum expiratory pressure (MEP). Effect sizes for changes in MIP and MEP were determined using Cohen's d statistic. Exploratory outcomes targeted motor function, and peak cough flow (PCF) was measured in the last 5 subjects. RESULTS: From pretest to posttest, all 8 subjects exhibited increases in MIP, and 7 of 8 showed increases in MEP. Effect size data reveal the magnitude of increases in MIP to be large in 4 (d≥1.0) and very large in 4 (d≥2.0), and effect sizes for increases in MEP were large in 1 (d≥1.0) and very large in 6 (d≥2.0). Across participants, pretest to posttest MIP and MEP increased by a mean of 19.6% (sd=9.9) and 16.1% (sd=17.3), respectively. Respiratory strength increases, particularly for the inspiratory muscles, were generally durable to 3-months detraining. CONCLUSIONS: These data suggest our 12-week RMT program results in large to very large increases in inspiratory and expiratory muscle strength in adults with LOPD. Additionally, increases in respiratory strength appeared to be relatively durable following 3-months detraining. Although additional research is needed, RMT appears to offer promise as an adjunctive treatment for respiratory weakness in LOPD.