Four patients with the clinical syndrome of primary progressive aphasia and a nonfluent aphasia profile were followed up over a period of 3 to 5 years. Extensive neuropsychological data for three patients revealed a progressive, quantitative decline of language with relative stability of memory, visuospatial skills, and reasoning. Comportment and most activities of daily living were preserved even when speech was unintelligible. Although several aphasia types may be associated with primary progressive aphasia, a nonfluent aphasia profile and phonemic paraphasic errors are most useful in differentiating it from the much more common clinical syndrome, "probable Alzheimer's disease." The clinicopathological correlates of probable Alzheimer's disease differ from those associated with primary progressive aphasia. Therefore, the clinical distinction between the two syndromes may be important for predicting the underlying pathophysiologic changes during the life of the patient.