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This study examined the impact of two neurological diseases on access to semantic knowledge and the status of semantic representations. Patients with Alzheimer's disease (AD) and Aphasia (APH) were compared with control groups using the supermarket fluency task. We measured several aspects: number of category names produced, number of categories sampled to produce the words, the number of words per category sampled, number of exemplars and kinds of errors recorded. Both AD and APH groups produced significantly fewer words on the fluency task than control groups. As compared with the APH and control groups, in the AD group verbal fluency was characterized by a tendency to generate more category names with fewer exemplars within a category. The findings are consistent with the view that a bottom-up disruption in semantic knowledge occurs in AD and a general semantic disruption occurs in Aphasia.