In an accompanying article (Jubran, et al., Am. J. Respir. Crit. Care Med. 155:906-915), we report that patients with chronic obstructive pulmonary disease (COPD) who failed a trial of weaning from mechanical ventilation developed worsening of pulmonary mechanics compared with patients who tolerated the trial and were extubated. We wondered whether the greater derangements in pulmonary mechanics in the weaning failure patients are evident ever before undertaking the weaning trial. We measured mechanics of the respiratory system, lung, and chest wall during passive ventilation at usual ventilator settings in 12 patients who went on to fail a weaning trial and in 12 patients who were successfully weaned. No differences in the resistances of the respiratory system, lung, and chest wall were observed between the two groups or when the resistances were separated into the components derived from ohmic resistance and viscoelastic behavior/time-constant inhomogeneities. Likewise, the groups did not differ in terms of static elastance and dynamic intrinsic positive end-expiratory pressure (PEEPi) of the respiratory system and the respective lung and chest wall components or in terms of dynamic elastances of the respiratory system and chest wall. The failure group had a higher dynamic elastance of the lung than the success group (p < 0.01), but the individual values showed considerable overlap among the patients in the two groups so limiting its usefulness in signaling a patient's ability to sustain spontaneous ventilation. Thus, mechanics of the respiratory system and its lung and chest wall components during passive ventilation did not satisfactorily discriminate between patients who failed a weaning trial and those successfully weaned, and, thus, are unlikely to be useful in signaling a patient's ability to tolerate the discontinuation of mechanical ventilation.