The majority of the patients with multiple sclerosis (MS) experience fatigue. Some observations indicate that fatigue and related manifestations concomitant with MS could be associated with an intracellular mild thiamine deficiency. We recruited 15 patients with MS who also experience fatigue and assessed the severity of the fatigue using the Fatigue Severity Scale. Although blood thiamine and thiamine pyrophosphate levels were within normal limit in all the patients, high-dose thiamine therapy administered orally or parenterally led to an appreciable improvement of the fatigue. The absence of apparent decrease in blood thiamine despite the presence of symptoms referable to a mild thiamine deficiency suggests that these patients may have a dysfunction of the mechanisms of intracellular transport or structural enzymatic abnormalities. The administration of large quantities of thiamine was effective in reversing the fatigue in MS, suggesting that the abnormalities in thiamine-dependent processes could be overcome by diffusion-mediated transport at supranormal thiamine concentrations.