Introduction: The aim was to elucidate the relationship between the annual changes in respiratory system impedance, measured by FOT, and lung function tests in patients with asthma. Methods: Between March 2011 and March 2012, asthma outpatients who attended Kobe City Medical Center West Hospital were recruited. Lung function tests, FOT were conducted every 6 months until March 2016. The relationships between annual parameter changes were evaluated. Results and conclusion: Sixty-four patients were completed this study. The median follow-up period was 55 months. At enrollment, although resistance showed no relationship with forced expiratory volume in one second (%FEV1), the reactance was moderately correlated with X5 (r = 0.524, r(2) = 0.275, <0.001), Fres (r = 0.498, r(2) = 0.248, <0.001) and ALX (r = 0.416, r(2) = 0.173, p = < 0.001). By contrast, the annual resistance change at 5 Hz (R5) was highly and significantly associated with%FEV1 change (r = -0.564, r(2) = 0.318, p < 0.001). Longitudinal changes in airway resistance and reactance measured by FOT might be useful for the assessment of lung function in patients with asthma. (C) 2017 Elsevier B.V. All rights reserved.