We compared "miniWright" peak flow (mWPF) readings with spirometric peak expiratory flow rates (PEFR) in 91 children, aged 8-15, exposed to ambient air at a summer camp in northwestern New Jersey. mWPF measurements immediately preceded spirometry, and mWPF-PEFR differences were regressed on spirometric PEFR by child. The ratio (mWPF-PEFR)/PEFR (mean +/- SE) was -0.027 +/- 0.028 (n = 91) (NS). When subdivided into PEFR ranges, the ratios were: -0.089 +/- 0.060 (n = 17) for 2-<4 Liters/second (Lps) (p = 0.04), -0.019 +/- 0.038 (n = 50) for 4-<6 Lps (NS); and -0.000 +/- 0.044 (n = 24) for 6-<8 Lps (NS). The mWPF values were also regressed on the average ozone (O3) concentration in the previous hour, by child. The mWPF response for O3 was -6.63 +/- 0.76 mL/sec/ppb, compared to -6.78 +/- 0.73 mL/sec/ppb O3 for PEFR. Thus: 1) mWPF, with an overall underestimation of approximately 2%, is a useful surrogate for spirometric values of PEFR (although, for the smallest children studied, it underestimated peak flows by approximately 9%), and 2) the portable miniWright peak flow meter is a convenient and effective tool for characterizing changes in PEFR associated with exposures to ambient O3.