Rationale: A new classification of patients based on the duration of liberation of mechanical ventilation has been proposed.Objectives: To analyze outcomes based on the new weaning classification in a cohort of mechanically ventilated patients.Methods: Secondary analysis included 2,714 patients who were weaned and underwent scheduled extubation from a cohort of 4,968 adult patients mechanically ventilated for more than 12 hours.Measurements and Main Results: Patients were classified according to a new weaning classification: 1,502 patients (55%) as simple weaning, 1,058 patients (39%) as difficult weaning, and 154 (6%) as prolonged weaning. Variables associated with prolonged weaning (>7 d) were: severity at admission (odds ratio [OR] per unit of Simplified Acute Physiology Score II, 1.01; 95% confidence interval [CI], 1.001–1.02), duration of mechanical ventilation before first attempt of weaning (OR per day, 1.10; 95% CI, 1.06–1.13), chronic pulmonary disease other than chronic obstructive pulmonary disease (OR, 13.23; 95% CI, 3.44–51.05), pneumonia as the reason to start mechanical ventilation (OR, 1.82; 95% CI, 1.07–3.08), and level of positive end-expiratory pressure applied before weaning (OR per unit, 1.09; 95% CI, 1.04–1.14). The prolonged weaning group had a nonsignificant trend toward a higher rate of reintubation (P = 0.08), tracheostomy (P = 0.15), and significantly longer length of stay and higher mortality in the intensive care unit (OR for death, 1.97; 95% CI, 1.17–3.31). The adjusted probability of death remained constant until Day 7, at which point it increased to 12.1%.Conclusions: Only patients who need more than 7 days for weaning have an increased mortality.