Twenty-seven patients with Alzheimer-type dementia (ATD) were treated with bright light therapy in the morning for four consecutive weeks. The cognitive state of each patient was evaluated with the Mini-Mental-State Examination (MMSE) and circadian rhythm with actigram before and after therapy for all of the patients and those of two groups divided by the severity criteria of the Clinical Dementia Rating. The therapy improved the circadian rhythm disturbances. Although the therapy caused no remarkable effects on dementia severity, it improved the MMSE scores, especially in the early stages of ATD. These results suggest that bright light therapy improved the circadian rhythm disturbances and then bettered the cognitive state in early-stage ATD.