Three patients with Alzheimer's disease of mild type received long-term treatment with THA at doses between 40 mg to 160 mg daily. This study describes the effects of THA on EEG, seen during long-term and high dose treatments. After short-term treatment, the changes of the mean frequency in the temporal and parieto-occipital regions varied between patients, but in the frontal region a significant increase of the mean frequency was seen in all patients. The EEG improvement seen in the frontal region was more significant than that for the other regions at low dose of THA (80 mg). During long-term treatment, we observed different types of EEG changes. The early improvement seen in two patients was temporary and reverted to the pre-treatment value. In a third patient who did not show marked improvement after short-term treatment, an improvement was seen in all regions after longer treatment. Treatment with high doses of THA (120 mg or 160 mg) induced a marked increase in the mean frequency not only in the frontal region but also in the temporal and parieto-occipital regions. Higher doses of THA seem to affect larger areas of the brain than lower doses of THA.