The purpose of this study was to examine the consequences of altering ventilator working pressure on airway pressure and flow characteristics during pressure support ventilation (PSV). A ventilator (Siemens Servo 900C) and single lung simulator were used, and graphic read outs, in triplicate, were taken at a variety of combinations of PSV, working pressure, lung compliance, and airway resistance. The graphic read outs were then analyzed for a number of "dependent variables," and multiple regression analyses were performed using working pressure, PSV level, compliance, and resistance as "independent variables." The results show that the relative impact of working pressure on airway pressure and flow will vary with other lung and airway characteristics; also, excessive working pressure results in significantly greater flow rates at 40 ms after onset of inspiratory flow and at maximum flow, greater ringing or overshoot in the circuit, reduced tidal volume and inspiratory time, and reduced area under the airway pressure curve. In conclusion, adjusting ventilator working pressure will significantly affect lung-ventilator interaction in a quantifiable fashion. Further, these findings support clinical evidence that working pressure and/or initial flow rate need to be individualized to ensure optimal airway flow and pressure characteristics.