Mesulam's (1982) report describing six patients with a slowly progressive aphasia without accompanying signs of dementia led to the recognition of a syndrome now known as Primary Progressive Aphasia (PPA). Many more patients have been described since Mesulam's description was published (see Westbury & Bub, 1997, for a review). However, the published literature is both unsystematic and incomplete, making it difficult to place the findings into a coherent theoretical framework. In addition, little previous work (see Mesulam, 1987) has specifically attempted to specify the difference between PPA and dementia of Alzheimer's type (DAT), although the two disorders are easily confused since many language deficits can masquerade on early presentation as memory or cognitive deficits. In this paper, the linguistic deficits of 11 PPA patients are analyzed, and contrasted with the linguistic deficits of a group of 11 DAT patients. Patients in both groups were tested using an extensive battery of language tests, the Psycholinguistic Assessment of Language Battery (Caplan & Bub, 1990; Caplan, 1992). We consider seven linguistic symptom clusters that differentiate the two groups.