Abstract There is substantial disagreement among clinicians concerning the etiologic significance of impaired nasal respiration. Conflicting views concerning the effects of breathing on facial growth suggest the need for a more quantitative approach to this important question. This investigation is the first in a series of studies representing a new direction for objectively assessing airway breathing. A model of the upper airway was used to study air movement under controlled conditions. The specific objectives were to determine the effects of airway size and shape on the aerodynamics of simulated breathing and develop a theoretical basis for predicting when breathing mode will change from nasal to predominantly oral. The following theoretical predictions are made on the basis of data generated from the model: 1. 1. A nasal airway cross-sectional area of less than 0.4 cm 2 may represent an inadequate airway in adults and some mouth breathing would be expected. 2. 2. The amount of adenoid obstruction must be very large to affect airway resistance. However, if airway resistance in the nose is high, large adenoids would present a serious airway problem and cause predominantly mouth breathing. 3. 3. When nasal airway resistance is high, the mouth will open approximately 0.4 to 0.6 cm 2. This shifts a significant amount of air orally and reduces airway resistance to a normal level. 4. 4. If morphologic changes are caused by airway impairment, other factors such as a large tongue, large tonsils, or a long, draping velum are probably significant contributing factors.