In order to evaluate the EEG activity during wakefulness in senile dementia of the Alzheimer type (AD) and Parkinson's disease (PD), a prospective controlled study was performed. We compared 6 AD and 11 PD patients with a control group of 12 patients with mild to moderate major chronic depression (DSM-III-R, 1987). The median of the frequencies and the power of the posterior dominant activity was obtained in the three groups using spectral analysis. The posterior activity had a frequency of 8.79 +/- 0.52 (mean +/- sd) in the control group, 6.65 +/- 0.80 (mean +/- sd) in the AD group and 7.69 +/- 1.39 (mean +/- sd) in the PD group. The experimental hypothesis that patients with AD and PD differ from controls in relation to the background activity (defined as abnormal < 8) was confirmed by the chi square test (p = 0.01) and the t test showed that the mean of the frequency of the posterior power was significantly lower in AD (p = 0.01) and PD (p = 0.05) patients, compared with the controls. The results indicate that this abnormality could be correlated with the degree of cortical damage and natural history of these disorders.