An important feature of Alzheimer's disease (AD) that differentiates it from normal aging is the deterioration of semantic memory. The purpose of this study was to determine if the poor performance of AD patients on tasks that require them to use semantic memory is related to their inability to identify specific semantic relations. This hypothesis was tested by comparing the reaction times and error rates of AD patients to those of the normal elderly and normal young in a sentence verification task. We found that AD patients were significantly slower and less accurate than the normal elderly and the young on aspects of the task that required the knowledge of specific semantic relationships. In a 2nd experiment, young normal participants performed the same task with a concurrent memory load. The results from Experiment 2 indicate that the performance of AD patients in this task cannot be attributed to a diminished working memory capacity.