Circadian rhythmicity was repeatedly determined in a patient with Alzheimer's disease by measuring his core temperature with a rectal thermistor and motor activity by an ambulatory activity monitor. The first recording, performed 9 years after he was diagnosed with Alzheimer's disease, showed well organized 24 hr circadian rhythm of core body temperature. The second recording, made four months later, showed very poor fit of core body temperature to 24 hour rhythm, but excellent fit with 36 hour rhythm. The third recording, made two months later, showed again good fit of core body temperature with 24 hour cycle. The last recording, which was performed 5 months later, showed almost complete disappearance of circadian rhythm of body temperature. These changes probably reflect gradual lengthening of the circadian cycle that at one point became extremely lengthened before returning to the 24 hr cycle.