The bispectral index (BIS) has been developed as a measure for monitoring the hypnotic drug effect. EEG processing results in a BIS scale from 0 to 100, where 100 represents an awake and responsive patient, and the scale decreases when hypnotics are administered. Here we describe two patients in whom the BIS decreased to nearly 0 during cardiac surgery. Postoperatively both patients showed hypoxic brain injury. There are several possible causes for a decrease in the BIS during surgery, including deep anesthesia, hypothermia and cerebral ischemia. In the present cases, cerebral hypoperfusion was the likely cause. During cardiac surgery, hemodynamic changes such as acute hypotension and cardiac arrest can cause cerebral ischemia. Cerebral ischemia develops most commonly during cardiopulmonary bypass (CPB). Therefore, the BIS may be useful for detecting severe cerebral ischemia during CPB, although it has some limitations as a cerebral monitor.