Circadian motor activity rhythms in 19 severely demented, institutionalized patients with Alzheimer's disease (AD) were evaluated with small, waist-worn electronic monitors which recorded 5-minute epochs for 48 to 72 hours. Controls were eight normal subjects of the same age (71 to 73 years) in a similar environment. As expected, computer-assisted analysis indicated more than twofold average increases in nocturnal activity and in the proportion of nocturnal to total daily activity in the AD patients. In patients (n = 8) with virtually constant pacing, daytime activity was markedly increased over that of normal controls; these "pacers" also had a significantly decreased amplitude of the circadian activity rhythm compared with controls. Moreover, AD patients showed a marked phase-delay, with individual afternoon maxima (acrophases) averaging 2.1 hours later than in controls (p less than 0.005). These findings quantitatively document clinical observations that AD patients, and especially a subgroup with pacing behavior, have markedly disturbed levels and modulation of daily locomotor activity. They accord with reports of altered circadian rhythms of endocrine and other physiologic parameters in such patients. Activity monitoring may represent a relatively simple, objective measure with which to characterize demented patients and to assess responses to treatment.