Circadian rhythms in elderly patients with severe dementia and behavioral disorders such as wandering, agitation and/or delirium were examined. The subjects consisted of 24 patients with dementia (5 with senile dementia of Alzheimer's type and 19 with multi-infarct dementia), aged 56-89 (means = 75.5 +/- 8.7) and 8 control patients without dementia or with dementia of slight degree, aged 65-81 (means = 75 +/- 5.4). The sleep-wake state of the patients was judged every hour by nurses over periods of 1-4 mo and recorded in the form of a sleep diary. Oral temperature was recorded for 4-7 consecutive days. For the treatment of sleep-wake rhythm disorders, social interaction with nurses was encouraged in addition to drug therapy. The patients showed various types of sleep-wake disorders such as reversed day-night rhythm or irregular sleep-wake rhythm corresponding to a decreased amplitude of the sleep-wake rhythm. Circadian rhythm of oral temperature was irregularly disturbed in 59.0% of the patients in the dementia group and in only 12.5% of the patients in the control group. The effects of treatment by enforcement of social interaction with nurses was effective in reducing behavioral problems and sleep-wake rhythm disorder in 30.0% of the patients tested. However, body temperature rhythm disorganization remained after the treatment. These observations indicate that behavioral disorders such as delirium, agitation or wandering in patients with severe dementia might be closely related to disrupted biological rhythms of sleep-waking and the autonomic system (body temperature).