BACKGROUND: Extubation is recommended to he performed at minimum pressure support (PSmin) during the pressure support ventilation (PSV). In field, physicians sometimes perform additional 1 hr T-piece trial to the patient at PSmin to reduce re-intubation risk. Although it provides confirmation of patient's breathing reserve, weaning could be delayed due to increased airway resistance by endotracheal tube. METHODS: To investigate the effect of additional 1 hr T-piece trial on weaning outcome, a prospective study was done in consecutive 44 patients who had received mechanical ventiIation more than 3 days. Respiratory mechanics, hemodymic, and gas exchange measurements were done and the level of PSmin was calculated using the equation (PSmin=peak inspiratory flow rate x total ventilatory system resistance) at the 15cm H2O of pressure support. At PSmin, the patients were randomized into intervention (additional 1 hr T-piece trial) and control (extubation at Psmin). The measurements were repeated at PSmin. during weaning process (in cases of intervention), and after extubation. The weaning success was defined as spontaneous breathing more than 48hr after extuintion. In intervention group, failure to continue weaning process was also considered as weaning failure. RESULTS: Thirty-six patients with 42 times weaning trial were satisfied to the protocol Mean PSmin level was 7.6 (+/-1.9)cm H2O. There were no differences in total ventilation times (TVT), APACHE III score nutritional indices, and respiratory mechanics at PSmin between 2 groups. The weaning success rate and re-intubation rate were not different between intervention group (55% and 18% in each) and control group (70% and 20% in each) at first weaning trial. Work of breathing, pressure time product, arid tidal volume were aggravated during 1 hr T-piece trial compared to those of PSmin in intervention group (10.4+/-1.25 and 1.66+/- 1.08 J/L in work of breathing)(191+/-232 and 287+/-217cm H2Os/m in pressure time product) (0.33+/-0.09 and 0.29+/-0.09 L in tidal volume) (P<0.05 in each). As in whole, TVT, and tidal volume at PSmin were significantly different between the patients with weaning success (246+/-195 hr, 043+/-0.11 L) and the Those with weaning failure (407+/-248 hr, 0.35+/-0.10L) (P<0.05 in each). CONCLUISON: There were no advantage to weaning outcome by addition of 1 hr T-piece trial compared to prompt extubation to the patient at PSmin.