Parietal and temporal lesions are known to impair binocular depth perception. Clinically meaningful impairment was therefore suspected due to early degeneration of these regions in Alzheimer's dementia. Results supported the cortical localization described in focal lesion studies. Deficient stereopsis was common in dementia (80%) but relatively uncommon in elderly normals (14%) and major depressives with cognitive symptoms (31%). Performance was unrelated to age, IQ, or severity of cognitive impairment. Testing involves minimal patient effort and time (1–2 min). Stereopsis examination may be useful in the diagnosis of early dementia, and may reduce false positives associated with depression, limited premorbid ability, or normal aging.