In 8 stable patients with chronic asthma, the reproducibility of peak expiratory flow rate (PEFR) measured and recorded 4 times per day for longer than 1 month at home and at work was assessed. Average %PEFRs of individuals through the periods observed were higher than 80%, and coefficients of variation were less than 10% regardless of the time measured. This suggested that a decrease in %PEFR more than 10% reflects early phase of deterioration of airways narrowing and frequent measurement of PEFR is essential for long-term management and for self-assessment of chronic asthma.