Diaphragmatic muscle performance during acute ventilatory failure due to Guillain-Barré syndrome and myasthenia gravis was assessed to evaluate (1) diaphragmatic function during weaning from ventilatory support and (2) diaphragmatic tension-time integral (TTdi) during ventilatory failure. We used a multilumen nasogastric tube and a pneumotachograph to measure transdiaphragmatic pressure per breath (Pdi), maximum transdiaphragmatic pressure (Pdimax), tidal volume (VT), and inspiratory time fraction during 74 spontaneous breathing trials in nine patients. Diaphragmatic performance was poor in all patients. The Pdi, Pdimax, and VT improved significantly, but values for Pdi and Pdimax remained low even after weaning. Improvement in Pdimax was the best predictor of recovery (r = 0.48; p less than 0.001). Maximal inspiratory force correlated with Pdimax (r = 0.48; p less than 0.005), but FVC did not. The TTdi rarely exceeded the expected fatigue threshold of 0.15 in spite of the patient's inability to sustain ventilation. Although our patients demonstrated diaphragmatic weakness, TTdi did not demonstrate diaphragmatic fatigue.