Magnetic resonance (MR) measures of muscle fat fraction are highly sensitive to disease progression and predict meaningful functional milestones in DMD. However, the minimum clinically important difference (MCID), “the smallest change that is important to patients” is not known. The purpose of this investigation was to estimate the MCID for vastus lateralis muscle fat fraction. As part of the multicenter ImagingDMD study, localized 1H-MR spectra (TE=108 ms; TR=3000 ms) were collected longitudinally from the vastus lateralis muscle in 180 males with DMD (0-8 annual follow up visits). Spectra were integrated and relaxation-corrected then used to estimate fat fraction (fat:(fat+water)). The MCID was evaluated using 1) the Standard Error Measurement (SEM) method, incorporating day-to-day reproducibility in 111 study participants; 2) the 1/3 Standard Deviation (1/3SD) method; 3) anchoring to loss of function; and 4) a survey of experts. The minimum detectable change (MDC) was estimated from the SEM and was 0.02. The SEM method yielded the lowest MCID estimate (0.02). The 1/3SD method was used to estimate MCID in subgroups of subjects defined by functional ability; in nonambulatory individuals, MCID was 0.04, in individuals who can walk but not rise from the floor, MCID was 0.05, and in individuals who can rise from the floor, MCID was 0.05. The annual change in FF associated with loss of ambulation was 0.10, which likely represents an upper bound to the MCID. Finally, median MCID for muscle fat fraction in a survey of 12 experts was 0.05. Thus, the range of MCID values for vastus lateralis MRS FF was 0.02 to 0.10, while MDC was 0.02. The time to reach an MCID was not examined, however a sustained annual change smaller than the MCID will eventually result in a clinically meaningful difference. These data provide context to interpret natural history and clinical trial results.