A computer-assisted technique based on the equation of motion of the respiratory system was used to measure inspiratory work of breathing in 11 patients during pressure support ventilation (PSV) and assisted, volume-cycled mechanical ventilation (AMV). During both modes of ventilation, patient work of breathing was calculated as the difference between the total work performed on the respiratory system (as predicted by the equation of motion) and the work performed by the ventilator. Patient work of breathing during AMV was also calculated as the difference between ventilator work measured during assisted and controlled mechanical breaths. By either method of work calculation, patient work of breathing during AMV was less than previously reported. In addition, when equal tidal volumes were delivered, there was no significant difference between the work performed by the patient during AMV and PSV. Patient work of breathing during PSV was found, however, to vary inversely with the level of pressure support. We conclude that: (1) patient work of breathing during AMV and PSV can be calculated using a computer-assisted technique based on the equation of motion of the respiratory system, and (2) depending on the amount of pressure support provided, patient work of breathing during PSV may be greater than, less than, or equal to the work performed during AMV.