A referred cohort of 67 clinically defined PPA patients were compared to 99 AD patients with formal language and nonverbal cognitive tests in a case control design. Language fluency was determined at the first and last follow up visits. Quantitation of sulcal and ventricular atrophy on MRI was carried out in 46 PPA and 53 AD patients. Most PPA patients (57%) are relatively fluent when first examined. Visuospatial and memory functions are initially preserved. Aphemic, stuttering, “pure motor” presentation, or agrammatic aphasia are seen less frequently. Later most PPAs become logopenic and nonfluent, even those with semantic aphasia (dementia). In contrast, AD patients were more fluent and had relatively lower comprehension, but better overall language performance. MRI showed significant left sided atrophy in most PPA patients. Subsequent to PPA, 25 patients developed behavioral manifestations of frontotemporal dementia and 15 the corticobasal degeneration syndrome, indicating the substantial clinical overlap of these conditions. Language testing, particularly fluency scores supported by neuroimaging are helpful differentiating PPA from AD. The fluent–nonfluent dichotomy in PPA is mostly stage related. The aphemic-logopenic-agrammatic and semantic distinction is useful, but the outcomes converge. (JINS, 2003, 9, 710–719.)