Airway cooling and drying has been proposed as a mechanism of exercise-induced asthma. Of interest in understanding the role of respiratory heat loss are the airway zones enduring the principal cooling and drying stresses. We have compared the axial rise in air temperature in the upper respiratory tract of asthmatics with that occurring in a laminar airflow steady-state model of convective heat transfer. The latter allowed an assessment of the contribution of airway geometry to the overall air warming process and gave some indication of the likely in vivo air temperature during hyperventilation, which due to the nature of our patients we could not measure directly. In vivo measurements were performed during a fibre-optic bronchoscopy. Eleven patients (67 years ±0·76) inhaled ambient air (23·2°C) and cold air (−17·5°C) nasally at a ventilation of 10 l min⁻¹. During cold air inhalation the air temperature of the pharynx was 32·7°C (1·0) and at the third-generation bronchi 37°C (0·5), whereas with ambient air these were 35·8°C (0·8) and 37·7°C (0·6), respectively. For the same inspired ambient air condition the corresponding air temperatures in the thermodynamic model were approximately 27°C and 32°C. The axial rise in air temperature in both the model and in vivo state were characterised by a rapid early warming phase regardless of airflow rate. We conclude that the region proximal to the pharynx will endure the most severe cooling during a hyperventilation challenge.