The purpose of the study was to identify personal characteristics that predict individual differences in acute FEV sub 1 response to ozone exposure. Response and predictor data were collected on 290 white male volunteers 18 to 32 yr of age who were each exposed to one of six concentrations of ozone between 0.0 and 0.40 part per million. The sample was divided into an exploratory sample of 96 and a confirmatory sample of 194 subjects. Exploratory analysis indicated that ozone, age, and several other variables explained a significant proportion of the variance in response. In the confirmatory sample, only age and ozone concentration predicted FEV sub 1 decrement. The authors concluded that for white male subjects age was significant predictor of response, with older subjects being less responsive to ozone. Furthermore, the authors demonstrated that exploratory analysis without control of type I statistical error rates may result in apparent findings that cannot be replicated.