Standardized exercise challenge tests, symptom scores and whole-blood eosinophil and basophil counts were made before and during the pollen season in 32 children suffering from hay fever (n = 16) or hay fever and asthma (n = 16). All participants developed rhinitis symptoms during the season. The hay fever group showed in addition a significant seasonal increase in cough score (but in no other asthma symptom) and in circulating eosinophils (P less than 0.01); mean exercise-induced bronchoconstriction (EIB) did not change despite a slight increase in a few subjects. The asthma group showed seasonal increases in EIB (P less than 0.001), asthma symptom score (P less than 0.002), and total eosinophil count (P less than 0.001). The increase in the latter was significantly higher (P less than 0.05) than that in the hay fever group. The relative basophil count remained unchanged in both groups. In conclusion, the hay fever group and the asthma group could be clearly distinguished with respect to EIB during natural pollen exposure. The significantly higher increases in EIB and circulating eosinophils observed in the asthma group might possibly be due to greater pollen antigen sensitivity in the asthmatics.