The driving behaviors of dementia patients have received little in the way of empirical scrutiny except through retrospective reports of crash rates. Understanding the driving errors of dementia patients and how they differ from those of normal older and younger drivers is important. This knowledge is basic to the development of road tests and scoring procedures to evaluate the driving competence of older, experienced drivers, especially those whose fitness to drive may have been compromised by a medical illness that alters their mental abilities. We have drive tested over 100 currently driving elderly patients with clinically significant cognitive decline (mostly diagnosed as the early stages of Alzheimer disease) and compared their performance with that of normal drivers. The study identified the types of driving errors that distinguish and differentiate the cognitively impaired group as well as a set of driving errors typical of both cognitively impaired and normal experienced drivers but differing in the number and severity of errors. A set of errors was also identified that did not differentiate the groups and should not be used in evaluating a person's competence to drive.