We compared expiratory tidal and minute ventilation during conventional and synchronized intermittent mandatory ventilation (IMV and SIMV) in 30 infants with respiratory failure. Identical ventilator settings were used during each mode in each infant. Tidal volumes of ventilator breaths were smaller during IMV than during SIMV (6.2 +/- 1.8 versus 7.4 +/- 1.9 ml/kg; p < 0.01). The coefficient of variation of tidal volumes was higher during IMV than SIMV for both ventilator (25 +/- 12% versus 15 +/- 8%) and spontaneous breaths (39 +/- 15% versus 24 +/- 10%, p < 0.001). Minute ventilation, however, was the same during both modes. During IMV, one infant breathed synchronously and two were phase-locked in asynchrony with the ventilator. The infants with a mixed interaction on IMV (27 of 30) had tidal volumes that depended on the phase of spontaneous breathing at the time of onset of each ventilator breath. Tidal volumes of IMV breaths that began during the first half of spontaneous expiration had the smallest tidal volumes (5.4 +/- 1.8 ml/kg, p < 0.01), followed by those that began during the last half of inspiration (6.4 +/- 1.8 ml/kg, p < 0.01). Thus, the synchrony produced by SIMV allowed the ventilator to deliver larger and more consistent tidal volumes than during IMV.