Home peak expiratory flow (PEF) measurements have become the cornerstone of asthma self-management plans. However, the cut-off values for changing treatment have not been formally tested. This study focusses on the possible overtreatment brought about by the different cut-off values and denominators currently employed. Data from 133 clinically stable asthmatic patients from a 2.5 yr follow-up study were analysed. The results showed that strict adherence to current criteria would lead to severe overtreatment, with up to 30% of clinically stable patients crossing into the lowest (red) zone at least once a year when personal best is the denominator and when it has not been limited to a defined time of day or to defined prior bronchodilator use. As expected, the passage of clinically stable patients into the lower zones became less frequent when cut-off values were sharpened and when time- and treatment-specific PEFs were used as the denominators. Strict adherence to commonly used peak expiratory flow cut-off values would lead to considerable overtreatment. In order to avoid overtreatment, the morning peak expiratory flow before any (bronchodilator) treatment should be related to the personal best peak expiratory flow measured under the same conditions. The choice of the right cut-off value will also depend on studies being performed to test the amount of undertreatment with a given value.