This study examined patients who fulfilled the National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association Work Group criteria for probable Alzheimer's disease (AD; n = 35) and controls for magnetic resonance imaging and psychometric data (n = 16) and for quantitative electroencephalogram data (n = 34). A cluster analysis performed on neuropsychological variables identified 2 AD subgroups: The AD1 group (n = 12) had impaired memory and executive functions but preserved verbal and visuospatial functions. The AD2 group (n = 23) had global impairment. The AD2 group had higher theta amplitude in the temporo-occipital, centroparietal, and frontal derivation and lower peak and mean frequency than the AD1 group or controls. Both AD groups had more severe memory deficits and clearly smaller hippocampal volumes than controls. This may implicate that the degree of damage in ascending activating systems differs in these subgroups, although the damage in the hippocampus is equal.