['To assess the ability of diaphragmatic ultrasound (US) to predict weaning failure in mechanically ventilated patients undergoing a first spontaneous breathing trial (SBT).', 'During a 4-month period, 67 consecutive patients eligible for a first SBT underwent US measurements of maximal diaphragmatic excursion (MDE) by a right anterior subcostal approach. Weaning failure was defined as either the failure of SBT or the need for resumption of ventilatory support for acute respiratory failure or death within 48h following successful extubation. The accuracy of diaphragmatic ultrasound and the Medical Research Council (MRC) score when predicting weaning failure was assessed via a receiver operating curve analysis.', 'The feasibility rate for the ultrasound measurements was 63%. Mean values of MDE were significantly higher in patients who succeeded at their first weaning attempt (4.1 ± 2.1 versus 3 ± 1.8cm, p = 0.04). Using a threshold of MDE ≤ 2.7cm, the sensitivity and specificity of diaphragmatic ultrasound in predicting weaning failure were 59% [39 - 77%] and 71% [57 - 82%] with an AUC at 0.65 [0.51 - 0.78]. There was no significant difference between MDE values and MRC scores for predicting weaning failure (p = 0.73).'] Is ultrasonographic diaphragmatic excursion inaccurate and not better than the MRC score for predicting weaning-failure in mechanically ventilated patients?