To study whether forced expiratory volume in 1 s (FEV1) for the diagnosis of bronchial reactivity by means of the methacholine inhalation challenge test could be appropriately replaced by simple measurements of peak expiratory flow rate (PEFR), 75 consecutively referred asthmatic children aged 6-15 y were examined during a symptom-free period. Their baseline FEV1 and PEFR values ranged from 70 to 130% (mean 99.1) and from 77 to 122% (mean 101.4) of those predicted, respectively. The methacholine inhalation challenge was performed with stepwise doubled cumulative doses and both FEV and PEFR were measured at each step. Of the 67 children who had a 20% reduction in either test, the fall in FEV was achieved after a lower dose of methacholine than the 20% fall in PEFR in 49 cases, after a higher dose in 15 and after the same dose in 3. There was a significant correlation (r = 0.56, p