In this study, we tried to clarify the characteristics of peak expiratory flow (PEF) measurement maneuver as a tool of daily monitoring severity of asthma in accordance with peak expiratory flow rate (PEFR) of forced expiratory maneuver. PEF did not differ when measured either in the standing or sitting position. However, obese individuals may have higher PEF when measured in the standing position and subjects with emphysematous change may have higher PEF when measured in the sitting than in the standing position. PEF did not differ when measured either with or without nose clip. Expiration time and volume in PEF measurement requires only one fifth and a half of that of forced expiratory flow volume measurement, respectively. In 36% of asthmatic patients in whom PEF is measured daily, PEF measurements were thought to be measured after inspiring at about 90% of forced vital capacity. Although PEF is a well tolerated useful clinical parameter in the management of bronchial asthma, it has different characteristics from peak expiratory flow rate (PEFR) of forced expiratory volume measurement.