The medium-term outcome of weaning from mechanical ventilation in COPD patients is not easy to anticipate because a respiratory fatigue may eventually develop. We evaluated the diaphragmatic function and the breathing pattern during 40 weaning trials on 15 patients ventilated after acute respiratory failure. We formed two groups according to the success (group B, n = 18) or failure (group A, n = 19) of the medium-term attempt (group A/less than 10 hours; group B/more than 12 hours). Provided the patients showed the classic weaning criteria (tidal volume greater than 5 ml/kg, respiratory frequency less than 30 breaths per minute, PaO2 greater than 50 mm Hg), the study of the breathing pattern did not allow differentiation between the groups. However, the transdiaphragmatic pressure (Pdi) and the Pdimax, which gave an indication of the power of diaphragm contraction, dropped early in the group that could not stand weaning, with an increase in the Pdi/Pdimax ratio. In addition, this same group showed a diaphragmatic dysfunction attested for by a frequent negative gastric pressure associated with or shortly preceded by an abdominal paradoxic motion.