The fundamental feature of delirium is disordered attention. Delirium is associated with a variety of medical illnesses including pneumonia, urinary tract infection, sepsis, meningitis, dehydration, congestive heart failure, uremia, liver failure, head injury, and postictal states. Increased mortality and morbidity are likely due to the seriousness of the underlying disease and the difficulty in caring adequately for very agitated patients who often cannot cooperate with treatments. Delirium can occur after an acute stroke and may even be the only observable neurological abnormality. The brain lesions in patients with abulia, when localizable, were located in the upper mesencephalic tegmentum, substantia nigra, medial thalami, striatum, and frontal lobes. Many of the lesions involved or interrupted projecting fibers to the frontal lobes. In contrast, when hyperactive agitated patients had focal brain lesions, the location was most often in the posterior portions of the cerebral hemispheres in the temporal, occipital, and inferior parietal lobes.