The determination of the minimum clinically important difference (MCID) is an important aspect of scale development. The Neurological Fatigue Index for multiple sclerosis (NFI-MS) was administered before and after expected change or stability in fatigue in 208 multiple sclerosis (MS) patients. The overall change scores of the NFI-MS accorded with perceived direction of change; importantly, no change was seen when none was perceived. Using the interval level NFI-MS scores, the largest MCID equated to 2.49 points on the Summary scale, 2.36 points on the Physical scale, 0.84 points on the Cognitive scale, 0.97 on the Diurnal Sleep scale and 1.95 on the Nocturnal Sleep scale. Our conclusion is that the NFI-MS responds as expected to changes in fatigue and has desirably small MCID scores.