Although life-saving, mechanical ventilation is associated with numerous complications. These include pneumonia, cardiovascular compromise, barotrauma, and ventilator-induced lung injury. Recent data from animal studies suggest that controlled mechanical ventilation can cause dysfunction of the diaphragm, decreasing its force-generating capacity--a condition referred to as ventilator-induced diaphragmatic dysfunction (VIDD). The decrease in diaphragmatic contractility is time-dependent and worsens as mechanical ventilation is prolonged. Evidence supporting the occurrence of comparable diaphragmatic dysfunction in critically ill patients is scarce, although most patients receiving mechanical ventilation display profound diaphragmatic weakness. Atrophy, fibers remodeling, oxidative stress, and structural injury have been implicated as potential mechanisms of VIDD. The decrease in diaphragmatic force that occurs during controlled mechanical ventilation is attenuated during assisted modes of ventilation. Whether the decrease in diaphragmatic contractility observed during controlled ventilation contributes to failure to wean from the ventilator is difficult to ascertain. Weaning-failure patients have reasons other than VIDD for respiratory-muscle weakness. Until we have further data, it seems prudent to avoid the use of controlled mechanical ventilation in patients with acute respiratory failure.