We investigated the respiratory heat and water loss in 24 asthmatic patients who performed bicycle exercise while inspiring air conditioned to varying temperatures and water contents. Measurements of peak expiratory flow and forced expiratory volume in 1 s were made at rest, during and after exercise to determine changes in airways resistance. Respiratory heat and water loss were measured using rapid thermistors and a mass spectrometer to measure inspired and expired temperature and water vapour pressure. Exercise-induced asthma (EIA) was enhanced as the inspired water content of the air was reduced. However, there was a wide variation in sensitivity to the loss of heat and water between individual patients. This variability could not be accounted for by differences in body size. When patients inspired air conditioned to body temperature and fully saturated with water vapour, EIA was significantly reduced; however, half the patients still had an attack of asthma following exercise which had induced no significant loss of heat or water. It is suggested that the bronchoconstriction induced by water loss from the airways during exercise may be due to a change in osmolarity in the respiratory tract fluid.