The physiologic effects of noninvasive ventilation (NIV) on work of breathing (WOB) and breathing pattern, respiratory-system mechanics, and hemodynamic function were examined via a literature review of clinical studies done between 1990 and 2008. Forty-one relevant studies were found; the majority examined patients with chronic obstructive pulmonary disease, whereas some also included patients with restrictive chest-wall disease or acute hypoxic respiratory failure. NIV reduced WOB in direct proportion to the level of inspiratory pressure-assist, and also by the ability of applied positive end-expiratory pressure (PEEP) to counter intrinsic PEEP. In general an inspiratory pressure-support level of 15 cm H(2)O and a PEEP of 5 cm H(2)O reduced most measures of WOB and inspiratory effort toward normal. When set to the same level of inspiratory pressure-assist, both pressure-support ventilation and proportional-assist ventilation effected comparable reductions in WOB. At high levels of inspiratory pressure-assist, NIV consistently increased dynamic lung compliance and tidal volume, and improved arterial blood gases. The hemodynamic effects of NIV are dependent upon the interplay between the type of mask, the level of inspiratory pressure-assist and PEEP, and the disease state. In general, patients with chronic obstructive pulmonary disease have a higher tendency toward decreased cardiac output at high levels of inspiratory pressure-assist, compared to those with acute lung injury.