OBJECTIVE To determine the effect of pressure-trigger (PT) and flow-trigger (FT) sensing on the work of breathing (WOB) during spontaneous, unsupported breathing in children receiving mechanical ventilation. METHODS Prospective clinical trial. METHODS Pediatric intensive care unit at a tertiary care children's hospital. METHODS Infants and children receiving mechanical ventilation for >24 hrs who were ready for extubation. METHODS During synchronized mandatory ventilation, WOB values for spontaneous, unsupported, non-synchronized mandatory ventilation patient breaths were measured using an esophageal balloon and FT or PT sensing on Servo 300 and PT sensing on Servo 900C ventilators assigned in random order. RESULTS Sixteen patients with a median age of 12.8 mos (25th-75th quartile [IQR], 8.1-38.2 mos), weight of 11.0 kg (IQR, 8.9-18.8 kg), and duration of mechanical ventilation of 7.5 days (IQR, 5.0-18.0 days) participated in the study. WOB for patient breaths using the Servo 300 FT sensing (0.8 J/L [IQR, 0.5-1.0]) was not significantly lower than WOB for PT sensing on Servo 300 (0.9 J/L [IQR, 0.4-1.3 J/L]) or the Servo 900C (0.9 J/L [IQR, 0.5-1.3 J/L]). However, when lung compliance was <0.75 mL.kg(-1).cm H(2)O(-1), WOB using the Servo 300 FT sensing (0.8 J/L [IQR, 0.6-1.1 J/L]) mechanism was significantly lower than those for PT sensing on both the Servo 300 (1.1 J/L [IQR, 0.9-1.3 J/L]) and Servo 900C (1.2 J/L [IQR, 1.1-1.5 J/L]) ventilators (p </=.017). CONCLUSIONS FT may not decrease WOB compared with PT for unsupported spontaneous breathing in children receiving mechanical ventilation. However, when lung compliance is decreased, FT sensing may be advantageous in decreasing the WOB.