Dementia is generally considered as rapidly progressive [rapidly progressive dementia (RPD)], in cases with overt cognitive impairment, established within months. Data about the relative frequency of underlying diseases in cases of RPD are few and extremely variable, depending on the clinical setting. We examined the relative frequency of the underlying causes of RPD, in a university tertiary referral center, in Athens. A series of consecutive patients presenting with RPD in a 3-year period was included. All patients received a comprehensive clinical, imaging, and laboratory evaluation. Of a total of 279 patients hospitalized for dementia diagnosis, 68 patients had RPD (37 males and 31 females). Mean age at onset +/-SD was 65.5+/-10.0. The most common cause of RPD was secondary dementias, accounting for 18 cases (26.5%). Alzheimer disease and frontotemporal dementia were almost equally represented, accounting for 12 (17.6%) and 11 (16.2%) cases, respectively. Vascular dementia, Creutzfeldt-Jakob disease, and various neurodegenerative diseases accounted for 9 cases each (13.2%). In a tertiary referral center, secondary dementias represented the most frequent cause of cases presenting with RPD. As a substantial number of these cases are potentially treatable, our finding reconfirms and underscores the importance of an exhaustive evaluation in any case presenting with RPD.