Cognitive decline in elderly schizophrenic patients is an important clinical symptom, but it is often difficult to analyze in detail due to the patient's original residual psychotic symptoms. In this article, the authors provide neuropsychological and neuropathological research information about cognitive decline in elderly schizophrenic patients, especially with reference to Alzheimer's disease (AD). Neuropsychological and neuropathological reports about cognitive impairments are reviewed. The effect of long-term antipsychotic medication upon cognitive function is also discussed. As a result, it is apparent that elderly schizophrenic patients often show cognitive impairments, however, such impairments do not have the characteristics of progressive degenerative illnesses such as AD, and the speed of their progress is very slow. Neuropathological studies have shown that AD brain pathology appears no more frequently among schizophrenic patients than in the normal population. Since making a diagnosis of AD means that the progressive deterioration not only of cognitive function, but also of physical ability, paralleling the degeneration of the central nervous system, can be expected within a few years and appropriate care will be required. One should be very cautious in adding a diagnosis of AD to elderly schizophrenic patients with cognitive impairments.