In order to devise a protective aid against bronchial obstruction induced by cold air, we have tested a breathing filter with heat and moisture exchanging properties. Nine asthma patients, who all had a history of cold-induced asthma, took part in exercise tests on an ergometer bicycle at a temperature of approximately -10 degrees C, without and with a breathing filter. Without a breathing filter, the maximum reduction in FEV1 was, on average, 36%. With the breathing filter, the maximum reduction in FEV1 was, on average, 11%. The difference was clearly significant (P < 0.001). A further five cold-sensitive asthmatics performed similar exercise tests at -10 degrees C on three occasions: 1) without and 2) with a breathing filter as above, and 3) with two breathing filters connected in parallel: one for inspiration and the other for expiration. Thus, no heat-moisture exchange could take place. The fall in FEV1 after provocation without a breathing filter and with parallel breathing filters was similar but attenuated when rebreathing took place through the breathing filter. The results confirm the theory that in cold/exercise-induced asthma, it is indeed the heat and/or water loss from the airways that triggers airway narrowing, and that a heat and moisture exchanging filter has a considerable protective effect and can be of value in the treatment of asthma.