Scientific data fail to support the widespread use of humidity and mist in respiratory therapy. A clear indication exists to provide humidification of inspired air in patients in whom the proximal airway is bypassed by endotracheal cannulation. Indications for the use of concentrated mist atmospheres in therapy of lung disease is less well documented, and too often the importance of adequate systemic hydration is overlooked. Theory supports the use of a highly humid or mist atmosphere in therapy of upper airway disease, but little benefit is derived from such therapy in diseases of the smaller airways and lung parenchyma. Danger of bacterial contamination and infection and an increase in airway resistance are significant hazards to the patient exposed to highly humid atmospheres.