Sleep impairment with agitated behavior is common in the demented elderly. The gravity of sleep and behavioral disturbances is evident from the fact that night time agitation is the primary cause of institutionalization of the demented elderly. Both problems are related to circadian rhythm changes and decreased exposure to zeitgebers. Bright light is known to be a strong zeitgeber of human circadian rhythm. Consequently, bright light has been emerged as a possible therapeutic alternative for the problems of sleep and behavioral disturbances. The purpose of this study was to examine the effect of evening bright light delivered by fluorescent lamps on female demented elderly with disturbed sleep patterns, accompanied by agitated behavior, in a community care facility. The study was conducted in cross-over trial with twenty-one subjects and outcome variables derived from tympanic temperature, sleep, agitation and cognition were analyzed. The results were as follows : 1. Temperature : Treatment effect was not statistically significant for the mean, peak, nadir, and amplitude (P>0.10). No statistically significant changes of the occurrence numbers of peak 4PM were found after treatment, but the occurrence numbers of nadir at 4 AM increased significantly (P=0.09) after treatment. 2. Sleep : Treatment effect was not statistically significant for the sleep period at night, total sleep time, wake after sleep onset, percentage of sleep during night, sleep time at night, wake up time in the morning. Nap time (P=0.01) and nap numbers(P=0.02) decreased significantly after treatment. 3. Agitation scores decreased significantly after treatment(P=0.00). 4. Cognition scores were not changed significantly after treatment(P=0.82). Evening bright light has significantly decreased the agitated behavior of the demented elderly, but were found to be effective on only a few outcome measures of temperature and sleep-activity rhythm, and ineffective on cognition. It can be concluded tat bright light is a potent therapeutic intervention for the sleep and behavioral disturbances of the demented elderly.