A study was undertaken to determine the frequency responses of eight peak expiratory flow (PEF) meters tested with forced expiratory trials having varying frequency content. Eight commercially available PEF meter models were exposed to forced expiratory flow simulations produced by a computer driven mechanical pump. PEF levels of 200, 400, and 600 liters/minute (l/min) were used, and for each level harmonic frequency limits of 2, 4, 8, 12, 16, and 20 hertz (Hz) were tested. Simulator output was confirmed using a pneumotachometer and pressure transducer, and also by measuring the displacement of the pump piston. PEF was overread in four models for waveforms with higher frequency content. From this study the authors conclude that PEF meters exhibit variable responses to PEFs of different frequencies, that frequency response characteristics should be included in device requirements and specifications, and that an upper PEF range of 12Hz and not 20Hz should be used with human subjects.