BACKGROUND: Depression is one of the most frequent neuropsychiatric comorbidities of dementia of Alzheimer type (DAT). The aim of the present study was to compare the quantitative EEG changes in patients with DAT with those in old age depression (OAD). METHODS: We recruited 39 subjects (mean age=65.4+/-5.9, 30 women, 13 DAT, 13 OAD, and 13 healthy controls) and performed the absolute and relative power spectral analyses for delta, theta, alpha, and beta activities. RESULTS: The DAT group showed higher frequency of delta in all the topographic areas than the OAD and controls (p<0.05). Delta activity was more frequent in the DAT than OAD group in the bilateral central (C3, C4) and temporal areas (T3, T4) (p<0.05). Compared with the controls, the DAT group showed decreased alpha activity in the left central (C3) and temporal (T3) areas (p<0.05). Beta activity was increased in the OAD group (O1 and O2), compared with that in the other two groups (p<0.05). CONCLUSIONS: Spectral analysis showed more prominent delta slowing in the DAT group, which is in line with more severe cognitive dysfunction in the DAT than the OAD group.