Reconnection to the ventilator for 1h following a successful spontaneous breathing trial (SBT) may reduce reintubation rates compared with direct extubation. However, the physiologic mechanisms leading to this effect are unclear. Does reconnection to the ventilator for 1h reverse alveolar derecruitment induced by SBT, and is alveolar derecruitment more pronounced with a T-piece than with pressure-support ventilation (PSV)? This is an ancillary study of a randomized clinical trial comparing SBT performed with a T-piece or with PSV. Alveolar recruitment was assessed by using measurement of end-expiratory lung volume (EELV). Of the 25 patients analyzed following successful SBT, 11 underwent SBT with a T-piece and 14 with PSV. At the end of the SBT, EELV decreased by -30%(95%CI, -37 to -23) compared with baseline prior to the SBT. This reduction was greater with a T-piece than with PSV: -43%(95%CI, -51 to -35) vs-20%(95%CI, -26 to -13); P< .001. Following reconnection to the ventilator for 1 h, EELV accounted for 96%(95%CI, 92 to 101) of baseline EELV and did not significantly differ from prior to the SBT (P= .104). Following 10min of reconnection to the ventilator, EELV wasted at the end of the SBT was completely recovered using PSV (P= .574), whereas it remained lower than prior to the SBT using a T-piece (P= .010). Significant alveolar derecruitment was observed at the end of an SBT and was markedly more pronounced with a T-piece than with PSV. Reconnection to the ventilator for 1h allowed complete recovery of alveolar derecruitment. ClinicalTrials.gov; No.: NCT04227639; URL: www. gov.