A previous study with a small sample (N = 39) showed a significant correlation between the cortical theta activity and the hippocampal volume in different stages of cognitive impairment in aged subjects. The recent study was aimed to replicate these results in a much bigger sample. The authors examined a sample of 121 right-handed subjects. The sample consisted of 37 healthy controls, 40 patients with questionable dementia, and 44 patients with mild dementia assessed by Clinical Dementia Rating. All subjects underwent EEG and brain MRI. Mean spectral power was calculated, and volume of hippocampal segments was measured. EEG theta power of the left and right hemisphere correlated significantly with the hippocampal volume on the left and right side in different stages of cognitive impairment. An increase of theta power was associated with decreased hippocampal volume. No other significant correlations were found for alpha or beta band power. No correlation was found between cortical theta and global brain volume. There seems to be a direct relationship between neuronal loss of the hippocampus and changed cortical theta activity for different stages of cognitive impairment in aged subjects.