Forty-four patients with dementia of the Alzheimer type (DAT), 44 elderly normal control (ENC) Ss demographically matched to the DAT group, 42 patients with Huntington's disease (HD), and 42 middle-aged normal control (MNC) Ss demographically matched to the HD group were administered letter and category fluency tasks. DAT patients showed an overproportional impairment on category than on letter fluency tasks, whereas HD patients were equally impaired. Analyses based on receiver operating characteristic curves revealed that category fluency correctly classified significantly more DAT and ENC subjects than did letter fluency, whereas the two fluency tasks did not differ in this respect for HD and MNC subjects. Results suggest that HD patients' failures on fluency tasks are caused by impaired initiation/retrieval capacities. In contrast, DAT patient's greater category than letter fluency deficits are primarily due to a breakdown in the structure of semantic knowledge.