A study was made to determine whether natural allergenic exposure modulates exercise-induced asthma. Eighteen asthmatic men, ten non-allergic and eight allergic to birch pollen, underwent heart rate-monitored outdoor exercise tests during both the cold winter season and in the spring, the birch pollen season. The mean fall in FEV1 after the outdoor exercise test increased in the allergic group from 17.3% in the winter to 27.6% in the spring, while it decreased in the non-allergic group from 31.6% to 22.4% respectively (P<0.01). Initial FEV1 and FCV values remained unchanged in both groups. The non-specific airway responsiveness to histamine did not change significantly in birch pollen allergic or non-allergic subjects during the spring, when compared with the winter values. We conclude that the exercise-induced asthma is aggravated in the birch pollen allergic asthmatics during the pollen season, when compared to the non-birch pollen allergic asthmatics, in whom the exercise-induced bronchoconstriction is attenuated as expected, because of the warmer and more humid weather in the spring.