A new pocket-sized ambulatory spirometer (meter) that measures FEV1 and PEFR was evaluated. The meter determines flow rate (V) from the differential pressure (P1-P2) across a sharp-edged orifice (V alpha square root of[P1-P2]). Ten meters were evaluated for accuracy using a computerized syringe to deliver the 24 ATS-recommended waveforms and nine scaled versions of a single waveform (PW#24). Within-meter reproducibility of FEV1 and PEFR was evaluated in two meters by passing five versions of waveform PW#24 through each meter 10 times. Reliability in the ambulatory setting was assessed in six meters on several occasions over a 10-week period using five versions of waveform PW#24. Results show that the 10 meters conform to the ATS accuracy specifications for PEFR with one or less errors and marginally outside these limits for FEV1 with four errors. For the nine versions of PW#24, the 95% confidence intervals indicate that the meter is accurate to within +/- 5.5% or +/- 15 L/min for PEFR and +/- 3.5% or +/- 0.12 L for FEV1. The mean within-meter coefficient of variation was 1.24% for FEV1 and 0.35% for PEFR. There was no significant change in meter accuracy or performance over the 10-wk reliability study. We conclude that the meter is suitable for use as an ambulatory spirometer for measuring FEV1 and PEFR.