Delirium is commonly defined as a transient organic brain syndrome characterized by concurrent disorders of attention, perception, thinking, memory, psychomotor behavior, and the sleep-wake cycle. One of the difficulties in studying delirium is that symptoms tend to fluctuate over the course of the day. Pre-existing organic brain disease appears to be a significant risk factor for the development of delirium, and numerous studies have shown a high rate of delirium in patients with cerebrovascular disease, Parkinson's disease, and Alzheimer's disease. The cognitive deficits associated with delirium have not been widely studied in a systematic, quantitative fashion. Following resolution of the frank delirium, documented cognitive deficits can be observed, and may persist in a diluted form for a period of months. Residual cognitive deficits may be due to a minimal and persistent confusion or to an underlying brain disorder.