The physical and physiological principles involved in the effective utilization of nebulization inhalation therapy are discussed. Objective data are presented supporting the use of mist tent therapy in the treatment of the chronic obstructive pulmonary lesion of cystic fibrosis. After three control periods the addition of mist tent therapy to an otherwise comprehensive treatment program resulted over the next 2 months in significant decreases in functional residual capacity, residual volume and ratio of residual volume to total lung capacity, and increase in maximum breathing capacity. These changes in the direction of improvement were maintained with continued use of the mist tent over the next year. A similar study involving elimination of mist tent therapy for 1 month and reinstituting it in the home treatment program revealed significant changes in pulmonary function in increased airway obstruction and overinflation during the period of elimination. These changes were reversed when mist tent therapy was reinstituted.