Fordadistrogene movaparvovec (PF 06939926) is an adeno-associated virus serotype-9 (AAV9) gene-replacement construct containing a mini-dystrophin being developed for DMD, which aims to restore functional protein to muscle. Magnetic resonance imaging (MRI) is an important non-invasive tool that can be used for monitoring disease progression and can inform future functional changes. Here we present 2-yr quantitative MRI measurements from 16 ambulatory participants with a genetic diagnosis of DMD from a phase 1b, multicenter, single-arm, open-label trial (NCT03362502). Fordadistrogene movaparvovec was administered as a single iv dose (2E14 vg/kg). An external reference (ER) cohort was derived from DMD participants (n=48) with ≥1 yr of randomized domagrozumab treatment (active→placebo, placebo→active, or active→active) from trial NCT02310763 and met the eligibility criteria for this trial. Whole thigh MRI scans were acquired at baseline, 6 months, 1-yr, and 2-yrs to evaluate changes in muscle volume (MV) and fat fraction (FF). At baseline, (mean±SD) thigh MV was 1,115±145cm3 and FF was 18.5±4.5% for fordadistrogene movaparvovec vs 1,150±282cm3 and 19.4±5.1% for ER participants. Fordadistrogene movaparvovec participants had a mean %change from baseline (CFB) in MV of 9.0±13.5% at 1-yr and 7.4±16.1% at 2-yrs. ER participants had a mean %CFB in MV of 2.7±8.3% after 1-yr and 2.4±12.1% after 2-yrs. Fordadistrogene movaparvovec participants had a mean CFB in FF of 0.6±2.8% at 1-yr and 3.3±3.2% at 2-yrs. ER participants had a mean CFB in FF of 1.6±2.6% after 1-yr and 3.3±4.1% after 2-yrs.Younger participants 6-7 yrs of age had the larger changes in %CFB in MV: fordadistrogene movaparvovec (n=6): 20.7±13.4% at 1-yr and 22.8±10.9% at 2-yrs vs ER (n=23): 4.5±8.4% at 1-yr and 6.0±13.6% at 2-yrs. After dosing with fordadistrogene movaparvovec, MRI scans of the thigh muscle and upper limb of study participants showed increases in MV and no clear difference in FF vs ER over 2 yrs. Fordadistrogene movaparvovec (PF 06939926) is an adeno-associated virus serotype-9 (AAV9) gene-replacement construct containing a mini-dystrophin being developed for DMD, which aims to restore functional protein to muscle. Magnetic resonance imaging (MRI) is an important non-invasive tool that can be used for monitoring disease progression and can inform future functional changes. Here we present 2-yr quantitative MRI measurements from 16 ambulatory participants with a genetic diagnosis of DMD from a phase 1b, multicenter, single-arm, open-label trial (NCT03362502). Fordadistrogene movaparvovec was administered as a single iv dose (2E14 vg/kg). An external reference (ER) cohort was derived from DMD participants (n=48) with ≥1 yr of randomized domagrozumab treatment (active→placebo, placebo→active, or active→active) from trial NCT02310763 and met the eligibility criteria for this trial. Whole thigh MRI scans were acquired at baseline, 6 months, 1-yr, and 2-yrs to evaluate changes in muscle volume (MV) and fat fraction (FF). At baseline, (mean±SD) thigh MV was 1,115±145cm3 and FF was 18.5±4.5% for fordadistrogene movaparvovec vs 1,150±282cm3 and 19.4±5.1% for ER participants. Fordadistrogene movaparvovec participants had a mean %change from baseline (CFB) in MV of 9.0±13.5% at 1-yr and 7.4±16.1% at 2-yrs. ER participants had a mean %CFB in MV of 2.7±8.3% after 1-yr and 2.4±12.1% after 2-yrs. Fordadistrogene movaparvovec participants had a mean CFB in FF of 0.6±2.8% at 1-yr and 3.3±3.2% at 2-yrs. ER participants had a mean CFB in FF of 1.6±2.6% after 1-yr and 3.3±4.1% after 2-yrs.Younger participants 6-7 yrs of age had the larger changes in %CFB in MV: fordadistrogene movaparvovec (n=6): 20.7±13.4% at 1-yr and 22.8±10.9% at 2-yrs vs ER (n=23): 4.5±8.4% at 1-yr and 6.0±13.6% at 2-yrs. After dosing with fordadistrogene movaparvovec, MRI scans of the thigh muscle and upper limb of study participants showed increases in MV and no clear difference in FF vs ER over 2 yrs.