Thirty-two asthmatic children, mean age 9.6 years (range: 6–14 years), were studied by oral challenge with acetylsalicylic acid (Aspirin), and their PEFR was recorded at 30 min intervals for 3 hr. They had been asthmatic for a mean of 7.1 years. Other allergic symptoms (urticaria, rhinitis or atopic dermatitis), were present in 81% of the patients, and a family history of atopy in 94%; the mean blood eosinophilia was 590 cells per mm3. In three children aspirin induced a fall in PEFR values less than 8% which was non-significant. In the group as a whole there was an increase in the PEFR values of 13.9%, 150 min after aspirin challenge. These values where subjected to statistical analysis (Kolmogorov–Smirnov, Student's and Wilcoxon tests), which showed this increase to be significant at a level of P= 0.001. Possible mechanisms involving prostaglandin synthetase inhibition by aspirin are discussed as an explanation for this increase.