OBJECTIVE: This paper presents dementia, as studied in the ancient Greek and Byzantine bibliography, aiming at clarifying whether there was an early differentiation between dementia and senile dementia.METHODS: Numerous references of the terms morosis, delirium (παραλήρημα) or "leros" (λήρος), and anoia were found in the writings of authors from the seventh century BC up to the fourteenth century AD. The references were studied and those referring to everyday use of the words were eliminated from the corpus, which is, thus, formed solely by the terms with medical connotations.RESULTS: The ancient history of dementia may be separated in two periods: before and after Posidonius, who differentiated two medical ontologies relating to dementia: dementia appearing due to old age called λήρος and dementia appearing due to other reasons called morosis. During the first period, we mainly have authors referring to dementia due to old age or due to cold black bile, defined as a deterioration of memory and reasoning. In all cases, the etiology and the clinical manifestations are presented while we make an attempt to correlate the ancient descriptions to modern medical entities.CONCLUSIONS: The study of the ancient texts proves that dementia was distinguished from senile dementia early in history. This differentiation was manifested even with a shift in the use of specific terminology. Finally, the attempt to correlate modern medical entities with dementia-related issues results in interesting hypotheses.