To evaluate the sensitivity of peak expiratory flow (PEF), obtained by portable peak flow meter, in detecting mild changes in airway caliber as assessed by forced expiratory volume in 1 sec (FEV1), we studied 184 subjects who underwent different bronchial challenge tests for suspected bronchial asthma. We measured FEV1 and PEF during bronchoconstriction induced by different stimuli: allergen, methacholine, toluene diisocyanate vapors, exercise, or distilled water inhalation; a total of 186 tests were examined. Before and at different times after challenge, FEV1 was measured, and immediately after, PEF was obtained by Mini-Wright or Assess Peak Flow Meter; each time FEV1 and PEF were taken as the best of three satisfactory tracings. The median FEV1 change from baseline value of all steps in the different challenge tests was 7.5% (range: 0-66%). The correlation coefficients between FEV1 and PEF percent changes in different challenge tests were low (Spearman's p: 0.27-0.69), with high scattering of the data. The concordance between classes of percent changes in FEV1 and PEF was also low (Cohen's weighted kappa: 0.28-0.42). In subjects with a FEV1 fall > 15% after challenge, the median PEF change after bronchoconstriction was lower than the corresponding FEV1 change [17% (0-52) vs. 27% (17-66)]. When different cutoff limits of PEF percent change were considered, the sensitivity of PEF to detect a significant change in FEV1 (15 or 20% change) during bronchoconstriction was low; specificity was in general higher than sensitivity. We conclude that PEF and FEV1 changes are poorly related during mild bronchoconstriction induced by different stimuli. The low sensitivity of PEF to detect mild changes in airway caliber may represent a limit in the use of PEF in the day-to-day monitoring of asthma.