We assessed language function, using a brief clinical Aphasia Battery and psychometric measures, in 150 subjects with senile dementia of the Alzheimer type (SDAT) and 83 elderly controls. Aphasia occurred only in demented subjects, and its prevalence increased with severity of dementia. Aphasia in mildly demented subjects was associated with both an earlier age of onset and more rapid progression of SDAT than in similarly demented nonaphasics. Language dysfunction in SDAT subjects was characterized by early decline in measures of comprehension and written expression, whereas other components, including oral naming, were less profoundly affected. Performance on the verbal psychometric measures, the Sentence Repetition and the Token tests, correlated strongly with Aphasia Battery scores and declined only minimally in nonaphasics, despite increasing dementia. We conclude that aphasia is a common feature of SDAT subjects and identifies a subgroup with more rapid progression of dementia. Furthermore, it represents language-specific dysfunction beyond the global cognitive impairment of SDAT.