Objective: The rapid shallow breathing index (RSBI) is relatively the best predictive parameter for initial assessment of readiness for the discontinuation of mechanical ventilation (MV) support. In this study, we aimed to determine the best pressure combinations that can predict successful RSBI closest to the values calculated in spontaneous ventilation (SV). Methods: Twenty-five mechanically ventilated patients were enrolled in the study. RSBI and other weaning parameters were calculated in different combinations (pressure support ventilation (PSV) 5 cm H2O / positive-end expiratory pressure (PEEP) 5 cm H2O; PSV 0 cm H2O/PEEP 5 cm H2O; PSV 5 cm H2O/PEEP 0 cm H2O; PSV 0 cm H2O/PEEP 0 cm H2O) before T-tube trial in all patients. Results: The mean age of the patients was 73±10 years. RSBI did not differ significantly between SV and other combinations. The best correlation with SV was found with 5 cm H2O PSV-0 cm H2O PEEP (p=0.0001, r=0.719) and the worst with 0 cm H2O PSV-5 cm H2O PEEP. RSBI showed no predictive value for weaning success. Respiration rate (f) was higher in failure than in the success group in PSV 0 cm H2O/PEEP 5 cm H2O and PSV 5 cm H2O/PEEP 0 cm H2O (p=0.030, p=0.030, respectively). f≤27 was considered as a predictive factor for spontaneous breathing trial (SBT) success (PSV 0 cm H2O/PEEP 5 cm H2O; sensitivity 93%, specificity 63%, PSV 15 cm H2O-PEEP 5 cm H2O; sensitivity 81%, specificity 75%). Conclusion: There was a good correlation between RSBI measured by T-tube and different pressure combinations.