This study was conducted in order to elucidate the functioning of the Central Executive System of Working Memory (WM) and to clarify the status of other cognitive functions in Dementia with Lewy bodies (DLB) and Alzheimer's disease (AD). Fourteen DLB, 22 AD, and 23 control subjects were assessed with the dual task paradigm and other cognitive tests. When compared with controls, DLB subjects performed more poorly in concurrent conditions on semantic WM tasks, and AD subjects performed more poorly on the spatial WM task. The DLB subjects had an inferior verbal span and AD subjects, an inferior recall on the CVLT. These data suggest relative impairments of verbal and semantic WM in DLB and relative impairments of spatial WM and verbal episodic memory in AD.