Butyrylcholinesterase is implicated in the pathology of AD. Selective inhibitors increase acetylcholine and improve cognitive function in animal models. In dementia with Lewy bodies, cholinergic activities are more affected than in AD. The authors report a highly significant association between temporal cortex butyrylcholinesterase activity and the rate of cognitive decline in a prospectively studied, autopsy-confirmed dementia with Lewy bodies series.