Five hours following an uneventful coronary artery bypass graft operation, an otherwise healthy 54-yr-old man developed a pneumothorax while his lungs were being ventilated in the recovery room. Neither arterial blood gas analysis, ventilatory variables, nor clinical examination had suggested this diagnosis, which was made subsequent to a chest radiograph taken as part of the assessment of hypotension. At the same time, the waveform of the pressure tracing from his pulmonary artery catheter changed inexplicably while attempting balloon inflation as part of the assessment of the hypotensive episode. In retrospect, the changes in the pressure tracing most likely were due to alterations in the pulmonary vasculature associated with the pneumothorax. These changes can be explained in terms of a well-known physiological model. If such changes are encountered in similar circumstances, a tension pneumothorax should be suspected.