For a small but important fraction of patients receiving mechanical ventilation, protracted dependence on a ventilator entails serious hazard, disability, and cost. Most such patients have a major catabolic illness, severe underlying lung disease, or profound neuromuscular weakness. As exemplified in the work of Yang and Tobin reported in this issue of the Journal,1 recent advances in our understanding of respiratory pathophysiology have facilitated decision making and the process of withdrawal (weaning) from mechanical ventilation. The need for ventilatory assistance stems from several sources, including psychological distress, refractory hypoxemia, and cardiovascular dysfunction. The most common cause, however, is an.