A number of reasons can be cited for performing a test that identifies patients early in their course who have fatal and currently untreatable neurological disorders. At this stage of illness there is clinical ambiguity. The patient, family, and physician are typically faced with a battery of negative test results and an ambiguous clinical impression that can lead to periodic repetition of tests that involve cost, inconvenience, potential morbidity to the patient, and lack of definitive diagnosis. An accurate test would lead to the avoidance of these low-yield, repetitive, and costly evaluations. In addition, such studies can identify homogeneous groups of individuals with degenerative disorders leading to dementia who could be enrolled in experimental therapeutic programs. In these programs therapies could be monitored in an objective and noninvasive fashion using positron emission tomography (PET). The magnitude of the health problems resulting from the dementing illnesses is great in terms of medical practice, economics, and family hardship. The number of individuals with these disorders is predicted to increase dramatically in the future. The ability to provide an accurate diagnosis and more clear prognosis early in the disease course should diminish ambiguity for patients, families, and physicians. Ample evidence is cited in this article to show that PET has the ability to provide such information objectively and noninvasively.