Monitoring and controlling respiratory drive and effort may help to minimize lung and diaphragm injury. Airway occlusion pressure (P0.1) is a non-invasive measure of respiratory drive. To determine: (1) the validity of ‘ventilator’ P0.1 displayed on the screen (P0.1) as a measure of drive, (2) the ability of P0.1 to detect potentially injurious levels of effort, and (3) how P0.1 displayed by ventilators compares to a ‘reference’ P0.1 measured from airway pressure recording during an occlusion (P0.1). Analysis of three studies in patients, one in healthy subjects under assisted ventilation, and a bench study with six ventilators. P0.1 was validated against measures of drive (electrical activity of the diaphragm and muscular pressure over time) and P0.1. Performance of P0.1 and P0.1 to detect pre-defined injurious effort was tested using derivation and validation datasets using esophageal pressure-time product as the reference standard. P0.1 correlated well with measures of drive and with esophageal pressure-time product (within-subjects R=0.8). P0.1 >3.5 cmH2O was 80% sensitive and 77% specific for detecting high effort (≥ 200 cmHO·sec·min); P0.1 <1.0 cmH2O was 100% sensitive and 92% specific for low effort (≤ 50 cmHO·sec·min). Area under Receiver-Operating-Characteristic Curves for P0.1 to detect potentially high and low effort were 0.81 and 0.92. Bench experiments showed a low mean bias for P0.1 compared to P0.1ref for most ventilators but precision varied; in patients, precision was lower. Ventilators estimating P0.1 without occlusions could underestimate P0.1. P0.1 is a reliable bedside tool to assess respiratory drive and detect potentially injurious inspiratory effort.