Although intrinsic, non-allergic asthma has a poor prognosis and requires a different treatment rationale than extrinsic asthma, there is no simple means of differentiating them at present. This study was designed to evaluate exercise-induced bronchospasm (EIB) as a differential test.Fifteen subjects with extrinsic asthma and six with intrinsic asthma were studied. Five minutes of treadmill exercise adequate to raise heart rate to 90-95% of maximum predicted was performed several times at 2-3 day intervals by each subject. VC, FEV1 and MMEFR were measured prior to exercise and at 1, 5, 10, 15 and 20 minutes after exercise.The percent fall in spirometry following 49 exercise periods in extrinsic asthmatics and 22 exercise periods in intrinsic asthmatics is seen below.Intrinsic asthmatics often responded to exercise with bronchodilation. Extrinsic asthmatics had greater exercise-induced bronchospasm during a significant pollen season.We concluded that EIB is more pronounced in extrinsic than in intrinsic asthma and may serve to differentiate the two entities.