Autonomic nervous system (ANS) failure may be an important complication in some neurological diseases. Conventional tests for the assessment of ANS functions during wakefulness cannot be used for many patients with dementia of the Alzheimer's type (DA) because of their poor cooperation. In 6 of 16 patients with presenile DAT we found signs of sympathetic cardiac dysfunction during sleep, measured as the decrease in body movement-related heart rate variation (Rbm). The study of Rbm during sleep seems to be a useful tool for ascertaining ANS function in DAT. The presence or absence of ANS dysfunction could be a more reliable criterion than age of onset for identifying subgroups of this disease.