The diagnosis of Alzheimer's disease must be considered in all subjects over 40 years of age whose mental functions have insidiously become altered and are progressively getting worse. The initial phase of the disease, which precedes dementia, is characterized by disorders of memory often associated with changes in personality and behaviour and with subtle disorders of speech, reasoning and abstraction ability and visuo-constructive capabilities. The results of neurological and paraclinical examinations are normal. This phase lasts from two to four years and may be diagnosed as "possible Alzheimer's disease". When the disease is established the patient has lost his autonomy and can only be kept at home with the help of his relatives. Dementia gradually becomes worse, with a combination of memory disturbance, aphasia-apraxia-agnosia syndrome, impaired intellectual faculties and disorders of behaviour. Computerized tomography shows evidence of cerebral atrophy. This phase lasts from three to six years and may be called "probable Alzheimer's disease". The terminal phase of the disease includes somatic lesions, and all mental functions are profoundly altered. Death occurs seven to ten years after the first symptoms. Histopathological data are necessary to make a diagnosis of "confirmed Alzheimer's disease", since there is no clinical, morphological, electrophysiological or radiological sign that is pathognomonic of that disease.