Posterior reversible encephalopathy syndrome (PRES) is a reversible neurologic disorder characterized by a variety of symptoms such as seizures, headache, vomiting, altered mental status, and visual disturbances. Bispectral index (BIS) monitor is a kind of electroencephalography and is widely used in anesthesia depth surveillance. In intensive care units, BIS has been used to survey sedation and predict prognosis of coma patients. Here, we report two cases diagnosed radiologically with PRES that had persistent consciousness disturbance resulted from severe life-threatening complications. In our patients, they progressed with different BIS values and different outcomes. We advise that BIS should be used in intensive care as an adjunctive monitor when patients' consciousness status could not be predicted.