A 37-year-old man was admitted to our hospital with acute phenobarbital poisoning. On arrival, he was in deep coma with respiro-circulatory depressions. The serum concentration of the agent was elevated to 149.04 μg/mL which was consistent with a lethal concentration level. He underwent a gastric lavage, administration of activated charcoal, urinary alkalinazation and bowel irrigation. Respiro-circulatory status was recovered rapidly, while the serum concentration of phenobarbital did not decrease smoothly. Although the concentration of the agent decreased to 77.07 μg/mL that should be a comatose level, BIS values were gradually elevated, and then eventually the patient regained his consciousness. Because he was a chronic user of Vegetamin-A containing phenobarbital, the serum level might not have been correlated with symptoms. BIS values were highly reflective of the consciousness level, so it could be a useful indicator for predicting the consciousness levels of patients in deep coma with acute poisoning from hypnotic agents.