Because large intersubject differences in the magnitudes of response to a single ozone (O3) exposure have been observed, we undertook to determine if this variability were due to differences in intrinsic responsiveness to O3 or to other factors. Thirty-two subjects were exposed to 1 of 5 O3 concentrations (0.12, 0.18, 0.24, 0.30, or 0.40 ppm), and each underwent one or more repeat exposures separated by from 3 wk to 14 months. Magnitudes of change for pulmonary function variables, respiratory rate and tidal volume, and for reported symptoms were compared for the repeated exposures. Changes induced in forced expiratory spirometric measurements were highly reproducible for as long as 10 months and for all tested O3 concentrations of 0.18 ppm or greater. This high degree of reproducibility indicates that the magnitude of response to a single exposure is a precise estimate of that subject's intrinsic O3 responsiveness. We conclude that the previously observed intersubject variability in magnitude of O3-induced effects is the result of large differences in intrinsic responsiveness to O3.