Two morphometric models differing in the tracheobronchial region, were compared in the present paper: Model 1 is based on the adult morphology of Weibel, assuming that all bronchial airways grow in equal proportion; while Model 2 adopts the adult structure proposed by Yeh and Schum, using measured airway dimensions in the right upper lobe as a function of age. Tidal volume and respiratory frequency also vary with age: while the breathing frequency decreases with rising age, tidal volume increases. Radiation doses in each bronchial airway generation were computed for the deep lying basal cells as well as for the more uniformly distributed serous (SMGS) cells, which are currently assumed to be the progenitor cells for bronchial carcinomas. Radiation doses to both target cells were significantly higher in the newborn than in the adult, for all simulated breathing patterns, showing the highest relative increase in upper bronchial airways. Comparing both tracheobronchial growth models, Model 1 predicts higher doses at early ages, but produced lower doses in the adult lung.