Objectives The purpose of this study was to establish whether intensive care unit (ICU) patients have impaired inspiratory muscle (IM) endurance immediately following weaning from prolonged mechanical ventilation (MV), and whether IM weakness is related to function or perceived exertion. Background Impaired IM endurance may hinder recovery from MV, however it is unknown whether this affects patients' function or perceived exertion. Methods Prospective observational study of 43 adult ICU patients following weaning from MV (>7 days duration). IM endurance was measured using the fatigue resistance index (FRI). Results IM endurance was impaired (FRI = mean 0.90, SD 0.31), with 37% scoring below 0.80. IM strength did not significantly correlate with function (r = 0.24, p = 0.12) or perceived exertion during exercise (r = -0.146, p = 0.37). Conclusions IM endurance is reduced in one third of patients, while IM weakness does not appear closely associated with function or perceived exertion immediately following successful weaning.