This study was designed to investigate the therapeutic efficacy of estrogen in female patients with dementia of the Alzheimer type (DAT). Fifteen DAT patients with a mean age of (x±SE) 71.9±2.4 years were treated with 0.625mg of conjugated equine estrogens orally twice a day for 6 weeks. Of the 15 DAT patients, 4 were diagnosed as mild, 7 as moderate and 4 as severe. The effects of estrogen on DAT patients were evaluated by psychometric assessments, behavior rating scales, regional cerebral blood flow (rCBF) measurement and quantitative EEG analysis. Psychometric assessments consisted of Mini-Mental State Examination (MMSE) and Hasegawa Dementia Scale (HDS). Dementia syndromes were evaluated by the CBS-Scale (GBSS) and Hamilton Depression Rating Scale (HDRS). During estrogen replacement therapy (ERT), the mean MMSE score (x±SE) increased significantly from 11.6±1.9 to 13.2±2.0 at 3 weeks (P<0.01) and 13.8±2.0 at 6 weeks (P<0.001). The mean HDS score increased significantly from 8.6±2.1 to 11.5±2.3 at 3 weeks (P<0.001) and 11.6±2.6 at 6 weeks (P<0.01). Significant improvements in the mean scores of the GBSS and HDRS were also observed in the estrogen-treated group, but not in the untreated control group with a mean age of 71.2±2.5 years (n=15). The rCBF was measured by single photon emission computed tomography (SPECT). ERT increased the mean rCBF significantly in the lower frontal region (P<0.01) and primary motor area (P<0.02) of the right hemisphere. The mean absolute power delta band values in both left and right frontal EEG (Fp₁ and Fp₂) (P<0.01) and theta₁ band values in Fp₂ (P<0.05) decreased significantly during ERT. It is inferred that ERT significantly improves cognitive functions, dementia symptoms, regional cerebral blood flow and EEG activity in female patients with DAT.