Reduction pneumoplasty may improve flow rates, comfort, and exercise tolerance in severe emphysema. The basis for improvement has not been systematically addressed. The major disability of emphysema stems from impairment of maximal expiratory flow-volume performance of the lung (MEFV). This requires the chest wall to operate at high volumes, which in turn severely compromises inspiratory muscle function. Clinical benefit, then, requires that MEFV performance improve so that the operating lung volume is reduced. This study presents theory and illustrative calculations. Removing nonventilating lung (e.g., bullae) simply displaces the MEFV curve down the volume axis. Removing ventilating parenchyma reduces both volume and maximal expiratory flow at iso-lung recoil pressure, and shortens the curve on the volume axis. The critical beneficial effect in both cases is reduction of the volume for a given limiting flow, VL (Vmax). Removing a given fraction of lung from the ventilating compartment is nearly as effective as removing it from the nonventilating compartment. Lowering of operating volumes benefits the strength, efficiency, endurance, and reserve of the inspiratory muscles and thus extends the metabolic scope of the emphysematous patient.