The specificity of anatomo-clinical observations were investigated on 776 out of 982 consecutive persons hospitalized at the University Psychiatric Clinic of Geneva. Discriminant function analysis shows that most of the anatomical classes (no dementia, senile dementia, Alzheimerized senile dementia and Alzheimer's presenile dementia, vascular dementia, combined dementia nnd undefined form of encephalopathy) are at least partially separable (less than 50% overlap). On the basis of anatomical criteria, Alzheimer's presenile dementia is not separable from Alzheimerized senile dementia, and senile dementia is not separable from combined dementia. Differentiation between the anatomical classes is improved by a preliminary analysis based on clinical diagnosis. Senile plaques account for 43.4% of the total variation between the anatomical classes. The coefficient of agreement between anatomical and clinical diagnosis is 0.27, which is highly significant. The diagnosis of senile dementia has a poor specificity, while the diagnosis of senile Alzheimerized dementia has a better one. The diagnosis of combined dementia has to be reserved for cases with a similar intensity of vascular and degenerative changes. Alzheimer's presenile dementia has a distinct dominant hereditary pattern and must be considered a separate entity. The high specificity of the diagnosis of Alzheimer's presenile dementia makes it possible to conduct epidemiologic and genetic surveys based on clinical data.