BACKGOUND: SIMV (synchronized intermittent mandatory ventilation) mode is comprised of a ventilator that intermittently enters the volume assist/control mode in conjunction with circuitry that allows for spontaneous ventilation by a demand flow system. There is additional work of breathing caused by the endotracheal tube and demand valve in SIMV. However, PSV (pressure support ventilation) has the ability to decrease work of breathing and to augment spontaneous breaths with a variable amount of inspiratory positive pressure with a clinician-selected level of inspiratory airway pressure. METHODS: To compare the initial weaning success rates and weaning periods between SIMV and PSV, we reviewed medical records of 103 intensive care unit patients, who had received mechanical ventilatory support and performed weaning. We compared the patients' characteristics, initial weaning success rates and ventilatory periods, weaning periods, weaning periods/ventilatory periods, ICU stays according to the weaning process (SIMV and PSV). And then we compared the same variables as the above between the initial weaning success group and initial weaning failure group. RESULTS: Patients' characteristics, ventilatory periods, weaning periods, ICU stays were similar, but there was significantly shorter weaning periods/ventilatory periods on PSV group. Initial weaning success rates according to the weaning process were similar. Ventilatory periods, weaning periods, weaning periods/ventilatory periods, ICU stays were significantly shorter on initial weaning success group. CONCLUSIONS: PSV could be used effectively as one of the weaning modes, and further studies are required about weaning criteria, weaning start time and weaning methods.