Because of clinical and neuropathological overlap between the characteristics of dementia of the Alzheimer type (DAT) and of a human thiamin deficiency syndrome (Wernicke-Korsakoff syndrome), thiamin pyrophosphate (TPP) dependent processes have been studied in DAT brain and other tissues. The activities of 3 TPP-dependent enzymes are reduced in DAT brain: transketolase (TK), the pyruvate dehydrogenase complex (PDHC), and the alpha-ketoglutarate dehydrogenase complex (KGDHC). Quantitatively, the most marked reductions are in KGDHC (to less than 20% of normal). In cultured skin fibroblasts, KGDHC activity is reduced to 50-60% of normal, TK activity to 80-90% of normal, and PDHC is normal. Structural and molecular studies of the DAT and non-DAT enzymes are in process. A lesion of KGDHC may be related to the pathogenesis of DAT. Treatment with large doses of thiamin has not been beneficial, but the data are not totally negative. Further studies of thiamin-dependent mechanisms in DAT seem justified.