Delirium (acute confusional states), a common and often overlooked psychiatric disorder, can occur at any age, but elderly persons are especially prone to develop it. In later life, it is often a conspicuous feature of systemic or cerebral disease and drug (notably anticholinergic) toxicity, and it may constitute a grave prognostic sign. Its development in a hospitalized patient may interfere with his or her management, disrupt ward routine, and cause medicolegal complications as a result of patient injury. Acute onset of a fluctuating level of awareness, accompanied by sleep-wake cycle disruption, lethargy or agitation, and nocturnal worsening of symptoms, are diagnostic. Early recognition of delirium and treatment of its underlying cause are essential.