Recent studies have shown some evidence that exercise-induced asthma (EIA) may not be entirely explained by respiratory heat loss (RHL). We investigated the interrelationship between heat exchange, exercise intensity and EIA. In order to differentiate between the effects of RHL and exercise intensity, we arranged for tests to be performed with the same RHL, but with different intensities of exercise and inspired air conditions. Each of 8 asthmatic children exercised twice in random order for 6 min on a cycle ergometer. One test consisted of exercise performed at a greater level of effort while breathing room air, mean (± SE) air conditions being 25.0 ± 0.4°C and 15.7 ± 0.2 mg H2O/L. The other test was performed at a lesser level of effort while breathing cold (0.0 ± 0.5°C) and dry air (O mg H2O/L). The mean ratio of minute ventilations in the 2 exercise tests was 1.78 ± 0.03, but the RHL was similar in both tests. The EIA after the exercise at the greater level was more severe than after the lesser level, the percent fall in FEV2 from baseline being 36 ± 7% and 21 ± 5%, respectively (p < 0.025). We conclude that the exercise level has a major role in determining the severity of EIA and that climatic conditions act as modifying factors.