BACKGROUND: Adaptive pressure control is a mode of mechanical ventilation where inflation pressure is adjusted by the ventilator to achieve a target tidal volume (VT). This means that as patient effort increases, inflation pressure is reduced, which may or may not be clinically appropriate. The purpose of this study was to evaluate the relationship between ventilator work output and patient effort in adaptive pressure control. METHODS: A lung simulator (ASL 5000) was set at compliance = 0.025 L/cm H2O and resistance = 10 cm H2O/L/s. Muscle pressure (Pmus) was a sine wave (20% inspiration, 5% hold, 20% release) that increased from 0‐25 cm H2O in steps of 5 cm H2O. The adaptive-pressure-control modes tested were: AutoFlow (Dräger Evita XL), VC+ (Puritan Bennett 840), APV (Hamilton Galileo), and PRVC (Siemens Servo-i and Siemens Servo 300). The target VT was set at 320 mL (Pmus = 15 cm H2O, inspiratory pressure = 0 cm H2O) to allow delivery of a realistic VT as the simulated patient demanded more volume. All measurements were obtained from the simulator. RESULTS: Patient work of breathing (patient WOB) increased from 0 J/L to 1.88 J/L through the step increase in Pmus. Target VT was maintained as long as Pmus was below 10 cm H2O. VT then increased linearly with increased Pmus. The ventilators showed 3 patterns of behavior in response to an increase in Pmus: (1) ventilator WOB gradually decreased to 0 J/L as Pmus increased; (2) ventilator WOB decreased at the same rate as Pmus increased but plateaued at Pmus = 10 cm H2O by delivering a minimum inspiratory pressure level of 6 cm H2O; (3) ventilator WOB decreased as in patterns 1 and 2 to Pmus = 10 cm H2O, but then decreased at a much slower rate. CONCLUSIONS: Adaptive-pressure-control algorithms differ between ventilators in their response to increasing patient effort. Notably, some ventilators allow the patient to assume all of the WOB, and some provide a minimum level of WOB regardless of patient effort.