Psychiatric features in dementia may reflect the coexistence of a previous „functional” mental disorder and dementia. To test this hypothesis we used the national Psychiatric Case Register (PCR) as a data source and (a) compared PCR data between phenomenologic subtypes of dementia patients with and without history of psychiatric hospitalization; (b) investigated in what way past psychiatric diagnoses relate to present dementia phenomenology. A small but significant portion of senile (31%) and arteriosclerotic (23%) dementia sufferers did have a documented history of psychiatric hospitalization. No significant or consistent differences concerning PCR demographic, clinical and clinical‐administrative variables studied were found when dementia patients of each subtype, with and without previous psychiatric hospitalizations, were compared. However, phenomenologic subtypes with delusions and depression in senile and arteriosclerotic patients were significantly associated with previous psychiatric „functional” diagnoses of a similar phenomenology. We suggest that, at least in a subgroup of senile and arteriosclerotic dementia patients, the psychiatric subtypes of delusions and depression might reflect the coexistence of two separate mental illnesses.