Using an actigraph and a long-term body temperature (BT) monitoring system, we simultaneously monitored rest-activity (R-A) and BT rhythms in patients with senile dementia of Alzheimer's type (SDAT; n = 20) or multi-infarct dementia (MID; n = 21) for 5-7 consecutive days. The SDAT group exhibited a well-organized BT rhythm with significantly higher amplitude compared with the MID group. The SDAT group also showed significant positive correlation between the total daily activity as well as percentage of nighttime activity and the degree of dementia, while no such tendency was observed in the MID group. The different properties of the biological rhythm disorders among the SDAT and MID groups possibly underlie their sleep and behavioral disorders.