Capnography is one of the basic monitoring techniques in day-to-day anesthesia practice that provides information not only regarding the patient's ventilation, circulation, and metabolism, but also regarding proper functioning of a closed-circle system. The authors report a case in which after endotracheal intubation the end-tidal capnometric reading rose very high, but the capnogram was not seen on the monitor. The unexpectedly high capnometric reading with absent waveform during intermittent positive pressure ventilation without any apparent cause and consequent delayed institution of corrective measures resulted in severe brain bulge. There was severe hypercarbia as a result of a malfunctioning expiratory unidirectional valve that allowed rebreathing. Retrospective retrieval of data showed that a fraction of inspired carbon dioxide was also high and the baseline was raised beyond the usual range of 0 to 40 mm Hg, giving the impression of an absent waveform on the existing scale. In conclusion, one should keep in mind the possibility of expiratory valve malfunction resulting from dislodgment while wheeling the anesthesia machine, the view becoming obscured as a result of condensation of water vapor on the under surface of the plastic case, and one should rely on the capnometric reading unless proved otherwise. Thus, one can prevent potential hazards of rebreathing.