The specific airway resistance (SRaw) is a relatively precise parameter of bronchial quality and in normal children its relation with body length, although very significant (P less than 0.001), is so poor that it can safely be disregarded (Fig. 4). Also the dispersion around the mean value remains stable at any body size (Fig. 4). Contrasting with the inverse relation between airway resistance (Raw) and lung volume, SRaw does not show any systematic variation with lung filling (Fig. 5). These data are consistent with the new formulas: SRaw = tg beta (PB - PH2O) and Raw = SRaw/TGV in which tg beta stands for relation between the plethysmographic box volume and breathing flow fluctuations (Fig. 2); PB - PH2O represents the ambient pressure corrected for water vapor pressure at body temperature; TGV is the thoracic gas volume at mean expiratory level. In disease SRaw varies more rapidly and markedly with any alteration of the airways than does Raw (Fig. 6). Consequently, a normal value for SRaw would indicate that both Raw and TGV must be in a normal range.