We believe that it is the loss of water per se and not heat loss that is the most important stimulus to exercise-induced asthma (EIA). The evidence to support this concept comes from a number of observations. First, EIA may occur when expired air temperature during exercise is close to body temperature and higher than that normally observed at rest. This suggests that EIA is not always associated with cooling of the airways. Second, there is evidence to show that the temperature of the inspired air is less critical than was previously thought. Providing that the water loss is the same, the airway response is reproducible even though the heat loss may vary because of differences in the temperature of the inspired air. Because expired water concentration is relatively constant, the airway response relates well to the water concentration of the inspired air. We believe that evaporative water loss from the airway mucosa induces a transient change in osmolarity in the epithelial fluid and that this change is a more potent stimulus to bronchoconstriction than is airway cooling. We believe this hypothesis accounts for many known facts about EIA and should merit further investigation by which it may be proved or disproved.