Behavioral and psychological symptoms of dementia (BPSD), especially aggressiveness, wandering, and sleep disturbance, are a major burden for caregivers. Daily sleep/wake patterns and wandering of institutionalized patients with Alzheimer's disease (AD) were visually monitored, and 34 patients who manifested wandering were selected and randomly classified into 2 groups: the risperidone group and the nonrisperidone group. After an administration of low-dose risperidone for the risperidone group, the BPSD were reassessed. The binding potentials of dopamine D2 receptor for preadministration and postadministration of risperidone were assessed using positron emission tomography (PET) for 1 case. After the use of risperidone, aggressiveness and wandering were reduced and the nighttime sleeping hours were increased. The PET revealed that the binding potential of dopamine receptor was increased after administration of the drug, associated with improved sleep/wake patterns and behavioral abnormality. Possible serotonergic modulation of dopaminergic function might explain the neurobiological basis of the effect of risperidone.