Carbon dioxide narcosis can produce apnea and coma, even though blood oxygen remains high enough to sustain life. We reported an unusual case of hypercapnia due to unrecognized recurarization with a high oxygen saturation on the pulse oximeter. A 74 year-old woman was scheduled for a spinal fusion due to a fracture of the lumbar vertebrae. During more than an hour after surgery, the oxygen saturation on the pulse oximeter was 99 or 100%. Neostigmine was repeatedly required due to only one twitch of train-of-four (TOF) stimulation at that time. Despite a return of four twitchs of TOF stimulation, the patient's level of consciousness deteriorated as a result of hypercapnia (PaCO2: 92.3 mmHg). Reventilation was required until this unusual carbon dioxide store had dissipated. We discuss the investigation, diagnosis and subsequent management of such an old patient and emphasize that the TOF stimulation was a reliable indicator of adequate recovery of neuromuscular function.