Epidemiology studies of the health effects associated with chronic exposure to ozone are constrained by a lack of validated exposure assessment methods that account for activity patterns and building ventilation, two potentially important predictors of ozone exposure. A pilot study was conducted to evaluate the impact of these parameters on an exposure model, to evaluate how reliably they can be reconstructed, and how this reliability affects estimates of past exposures to ozone. Fourteen subjects completed time-activity diaries and wore personal passive monitors during summer 1996 and reconstructed their activities 1 year later in a questionnaire. These data were used in a microenvironmental exposure model and the results were compared to measured personal exposure. A model using activity patterns, building ventilation, and location-specific ambient ozone data produced mean exposure estimates 18.9 +/- 1.5 ppb ozone greater than personal measurements. A simpler model that omitted variation in indoor ventilation produced exposure estimates 5.0 +/- 1.3 ppb ozone greater than mean measured exposures. The questionnaire was a reliable survey method for reconstructing past activity patterns. However, detailed histories of building ventilation may not be required to reconstruct reliably past exposures to ozone. The simple model was more precise and more practical for implementation in large epidemiological studies of health effects related to chronic exposure to ambient ozone.