The flexible fiberoptic bronchoscope is used increasingly often as a multipurpose instrument in critical care medicine. In poor risk patients who need continuous mechanical ventilation, rigid open tube bronchoscopy is a problem. With the flexible fiberoptic bronchoscope, however, diagnostic and therapeutic procedures can be carried out without interruption of ongoing mechanical ventilation. This procedure offers the possibility of bronchoscopy with reduced risk in debilitated patients. However, in these critically ill patients, the cardiopulmonary system is functioning at the borderline of its ability. Therefore, even the small changes in ventilation pattern caused by flexible fiberoptic bronchoscopy (FFB) may in some cases cause dangerous cardiopulmonary distress. For example, changes of intrabronchial pressure, tidal volume, PaO2, PaCO2 and cardiac output may be caused by the procedure. Further, it is of great importance to restrict suction through the instrument to short periods to avoid dangerous alterations in the ventilation perfusion relationship. Since serious complications may occur, it is mandatory that the bronchoscopist be aware of the potential pathophysiologic effects of FFB during mechanical ventilation of critically ill patients.