Thiamine-dependent processes are diminished in brains of patients with several neurodegenerative diseases. The decline in thiamine-dependent enzymes can be readily linked to the symptoms and pathology of the disorders. Why the reductions in thiamine linked processes occur is an important experimental and clinical question. Oxidative stress (i.e. abnormal metabolism of free radicals) accompanies neurodegeneration and causes abnormalities in thiamine-dependent processes. The vulnerability of thiamine homeostasis to oxidative stress may explain deficits in thiamine homeostasis in numerous neurological disorders. The interactions of thiamine with oxidative processes may be part of a spiral of events that lead to neurodegeneration, because reductions in thiamine and thiamine-dependent processes promote neurodegeneration and cause oxidative stress. The reversal of the effects of thiamine deficiency by antioxidants, and amelioration of other forms of oxidative stress by thiamine, suggest that thiamine may act as a site-directed antioxidant. The data indicate that the interactions of thiamine-dependent processes with oxidative stress are critical in neurodegenerative processes.