An etiologically nonspecific syndrome characterized by concurrent disturbances of consciousness and attention, perception, thinking, memory, psychomotor behaviour, emotion, and the sleep-wake cycle. It may occur at any age but is most common after the age of 60 years. The delirious state is transient and of fluctuating intensity; most cases recover within 4 weeks or less. However, delirium lasting, with fluctuations, for up to 6 months is not uncommon, especially when arising in the course of chronic liver disease, carcinoma, or subacute bacterial endocarditis. The distinction that is sometimes made between acute and subacute delirium is of little clinical relevance; the condition should be seen as a unitary syndrome of variable duration and severity ranging from mild to very severe. A delirious state may be superimposed on, or progress into, dementia.