Background: Measurement of peak expiratory flow rate (PEFR) in a follow-up examination for a chronic airway disease is useful because it has the advantages of being a simple measurement and can be repeated during examination. The aim of this study was to examine the annual decrease of PEFR in asthma and chronic obstructive pulmonary disease (COPD) patients and to confirm the factors which influence this decrease. Methods: From May, 2003 to September, 2010, the annual decrease of PEFR was obtained from asthma and COPD patients attending an outpatient pulmonary clinic. PEFR was measured using a Mini-Wright peak flow meter (Clement Clarke International Ltd. UK), and we conducted an analysis of factors that influence the change of PEFR and its average values. Results: The results showed an annual decrease of $1.70{pm}12.86$ L/min the asthmatic patients and an annual decrease of $10.3{pm}7.32$ L/min in the COPD patients. Age and $FEV_1$ were the predictive factors influencing change in asthma, and $FEV_1$ and smoking were the predictive factors influencing change in COPD. Conclusion: We confirmed the annual decreasing PEFR in patients with chronic airway disease and identified factors that work in conjunction with $FEV_1$ to influence the change.