Helium (He) has a much lower density than air. Thus, in turbulent flow conditions, helium-oxygen (He-O2) mixtures decrease resistance to flow compared to air-O2 mixtures. Theoretically, this effect could be beneficial in acute respiratory failure resulting from increased airway resistance, such as occurs in upper airway obstruction, asthma, and chronic obstructive pulmonary disease (COPD). Small uncontrolled series and case reports have indeed shown rapid albeit transient clinical improvement in upper airway disease. In asthma, studies have documented an increase in peak expiratory flow rate, an improvement in blood gases, and a reduction in pulsus paradoxus. In COPD, improved blood gases, expiratory flow, decreased dynamic hyperinflation, and reduced work of breathing and dyspnea have been observed. However, even though these encouraging results suggest that He-O2 should be included in the therapeutic options in such situations, there is no convincing data yet as to the exact place this gas mixture should occupy in treatment algorithms. This issue should clearly be explored further since He-O2 is a costly treatment not entirely devoid of potential deleterious effects mainly stemming from the ability of its low density to interfere with the proper functioning of mechanical ventilators.