PURPOSE:Bronchial hyperresponsiveness (BHR) to methacholine and exaggerated peak expiratory flow (PEF) variability are hallmarks of asthma. The aims of our study were to evaluate the relationship between PEF variability and BHR to methacholine and which PEF index correlates best with BHR to methacholine. METHODS:Methacholine challenge test was performed in 73 children having mild to moderate asthma. Those subjects recorded PEF morning and evening before and after bronchodilator for 2 weeks. The response to methacholine challenge was measured by PC20 (provocative concentration causing a 20% fall in FEV1), and seven different PEF variability indices(including prebrochodilator amplitude%mean, postbronchodilator amplitude%mean, standard deviation%mean) were calculated. RESULTS:Geometric mean of methacholine PC20 was 1.7 mg/mL. All PEF variability indices were associated with BHR to methacholine. Among PEF variability indices, two indices showed the best correlation with BHR to methacholine: standard deviation%mean (r=-0.45, P<0.001) and postbronchodilator amplitude%mean (r=-0.42, P<0.001). CONCLUSION:Standard deviation%mean provided the strongest correlation with BHR to methacholine. Meanwhile, postbronchodilator amplitude%mean which is counted easily and is more intuitively visualized manifested similar correlation as standard deviation%mean. Methacholine challenge test and PEF variability were correlated significantly but weakly; therefore we supposed that they do not reflect the same pathophysiological process in the airways.