Evaluated 66-yr-old male monozygotic twins with probable Alzheimer's disease (AD) with standard neuropsychological and psychiatric procedures. Brain anatomy was assessed by x-ray computed tomography (CT) and magnetic resonance imaging (MRI); local cerebral glucose metabolism was determined by positron emission tomography (PET). Both patients had a 2-3 yr history of impaired functioning. Twin A was more impaired clinically and showed greater atrophy on CT and MRI scans. Neuropsychologically, both twins had deficits in memory and abstract reasoning, with less impairment in verbal abilities. Twin A demonstrated bilateral deficits in tactile functions and a left hemi-inattention. Twin B exhibited right-sided sensorimotor deficits. Both twins had decreased metabolic rates, and differences in regional metabolism paralleled patterns of behavioral deficits.