Synchronized intermittent mandatory ventilation (SIMV) intermixes assisted and spontaneous breaths. Its ability as a weaning technique has been questioned on the basis that patients show little adaptation to ventilator assistance. We studied inspiratory effort and patient-ventilator interaction at different levels (SIMV, 100, 50, and 0%) of flow-triggered SIMV versus pressure-triggered SIMV in patients during the weaning period. The two triggering systems were evaluated during constant flow and constant pressure mandatory SIMV breaths. Inspiratory effort was estimated as the esophageal pressure time product (PTP) per breath (PTP/b) and per minute (PTP/min). The PTP/b and PTP/min of both mandatory and spontaneous breaths were significantly lower during flow triggering than during pressure triggering SIMV, irrespective of the ventilatory mode. During pressure-triggered SIMV PTP/b and PTP/min were identical for mandatory and spontaneous breaths, whereas during flow-triggered SIMV PTP/b and PTP/min were significantly lower for mandatory than for spontaneous breaths. This difference was greatest when flow triggering and constant pressure ventilation were associated. These data show that flow triggering reduces inspiratory effort during both mandatory and spontaneous SIMV breaths and obtains a better patient-ventilator interaction.