Of the remaining 80% of dementia patients, their dementia is considered irreversible. Irreversible dementia is still treatable, however the patient's cognitive abilities will not return to their pre-dementia levels. Alzheimer's is the cause of 66% of irreversible dementias. Muti-infardt dementia (MID) contributes towards 15.5% of all reversible dementias. The remaining 18.5% have several different causes, such as Parkinson's, Huntington's, alcohol, and mixed vascular and Alzheimer dementias.

There are variations in morbidity indices among the genders and races. For example, dementia tends to be more prevalent among women and African Americans. In Japan the prevalence of AD is lower than that reported in Europe which is similar to the US. In Japan, there is a higher prevalence of vascular dementia than among Europeans, although this difference tends to diminish with the oldest old. This may be due to increased mortality among those with vascular dementia at relatively younger age. In Japan, hypertension is more common than among Western Europeans, and treatment may be less aggressive than in Europe.