BACKGROUNDS: Assessment of the presence and degree of reversibility of airflow obstruction is clinically important in patients with asthma or chronic obstructive pulmonary disease. The measurement of peak expiratory flow(PEF) is a simple, fast, and cheap method to assess the severity of obstruction and its degree of reversibility. Assessing the reversibility of airflow obstruction by peak expiratory flow(PEF) measurements would be is practicable in general practice, but its usefulness has not been well investigated. We compared PEF and FEV1 in assessing reversibility of airflow obstruction in patients with chronic obstructive pulmonary disease or asthma and developed a practical criterion for assessing the presence of reversibility in general practice. METHODS: PEF measurements were performed (Spirometry) in 80 patients(aged 24-78) with a history of asthma or chronic obstructive lung disease before and after the inhalation of 200 mg salbutamol. The change in PEF was compared with the change in forced expiratory volume in one second(FEV1). Reversible airflow obstruction was analysed analyzed according to American Thoracic Society(ATS) criteria. RESULTS: When defined as a 12% A 12% increase above the prebronchodilator value and a 200ml increase in either FVC or FEV1 reversibility was were observed in 45%(36) of the patients. Relative operating characteristic(ROC) analysis showed that an absolute improvement in PEF of 30l/min gave optimal discrimination between patients with reversible and irreversible airflow obstruction(the sensitivity and specificity of an increase of 30l/min in detecting a 12% increase above the prebronchodilator value and a 200ml increase in either FVC or FEV1 were 72.2% and 72.7% respectively, with a positive predictive value of 68.4%) CONCLUSIONS: Absolute changes in PEF can be used to diagnose reversible airflow obstruction.