Primary degenerative dementia (PDD) and multiinfarct dementia (MID) are the two most common categories of cognitive decline in old age. The definitions of these two clinical entities are currently based on clinical evaluation and on the exclusion of other underlying causes, and still lack a consensus. The DSM-III-R criteria are widely used for the diagnosis of dementia. However, their role in the differentiation between PDD and MID has not been thoroughly examined. A consecutive series of 98 demented patients who met the DSM-III-R criteria for dementia were admitted to a clinical study. Upon evaluating their type of dementia according to these criteria, 53 patients could not be diagnosed either as having PDD or MID. The DSM-III-R criteria for these two types of dementia are critically reviewed. Proposed modifications, aimed at refining their differential diagnostic role, are presented, enabling better allocation of demented patients into PDD, MID, or intermediate groups.