The maximal pressure that can be generated during an inspiratory effort against an occluded airway serves as an index of respiratory muscle strength. We devised a method that permits accurate measurement of MIP, with near maximal values, and does not require patient cooperation. Twenty-two critically ill intubated patients performed MIP maneuvers before and after coaching. For the initial 11 patients, MIP was measured after the airway was occluded in 20 s with a one-way valve that permitted only exhalation. In the latter 11 patients, DS (approximately 1/3 VT) was added in an effort to increase respiratory drive before the noncoached MIP maneuver. We found no significant difference between coached and noncoached MIP maneuvers when P0.1 during the first 100 ms of inspiratory efforts prior to the noncoached MIP maneuver was greater than 2 cm H2O. Thus, MIP can be reliably measured in critically ill patients with or without coaching.