Introduction STUDY of pulmonary vascular resistance following thoracotomy in the human has largely been confined to the late postoperative period.1 We have measured pulmonary vascular resistance (PVR) preoperatively and in the early postoperative period. A marked elevation at this time is the reason for this presentation. Our interest in the changes in PVR immediately following thoracotomy was stimulated in the early days of open-heart surgery at our institution, when, in error, we operated on a patient with primary pulmonary hypertension. The death of this patient 48 hours later from unrelenting pulmonary hypertension (as gathered by indirect means) in a clinical state of low cardiac output implied an elevation of the PVR. A similar mode of death in the first few postoperative days was experienced in certain patients with high resistance type of ventricular septal defect. Could this be due to an increase in PVR following thoracotomy? If this same