Episodic (recall of passages) and semantic (letter and category fluency) memory tasks were administered to Alzheimer's Disease (early stages), Huntington's Disease (HD), and alcoholic Korsakoff patients matched for overall severity of dementia. Although all three patient groups were severely (and equally) impaired on memory for passages, only the Alzheimer and Korsakoff patients emitted numerous intrusion errors. On the fluency tasks, the performance of the mild Alzheimer patients was distinguishable from that of the other two patient groups. On both fluency tasks, the HD and Korsakoff patients demonstrated severe and moderate deficits, respectively, whereas the mild Alzheimer patients were impaired only on the category fluency task. As with the episodic memory test, the Alzheimer and Korsakoff patients made more perseverative errors than did the HD patients on letter fluency. These findings suggest that Alzheimer and HD patients' impairments on episodic and semantic memory tasks reflect different underlying processes. The performance of Alzheimer patients is affected by their language dysfunction and an increased sensitivity to proactive interference; the deficits of the HD patients appear due to a general retrieval problem. Similarities in the error patterns (i.e., perseveration errors) of Alzheimer and Korsakoff patients are discussed with regard to recent neuropathological findings.