Effects of ozone exposure on outdoor farm workers in the Fraser Valley of British Columbia, Canada, were investigated. Fifty-eight workers underwent spirometry daily before and after each 8- to 14-h workday from June 23 to August 26, 1993. The mean daily maximum (1 h) ambient ozone concentration was 40 ppb (range: 13 to 84 ppb). Concentrations of acid aerosols and fine particulates, potential confounders of ozone effects, were very low. In individual regressions of evening FEV1 and FVC on maximum daily ozone concentration, 47 of 53 workers with valid data (46 of 53 for FVC) had negative slopes. The average slopes (weighted by the inverse SE of the regression coefficients) were -3.3 and -4.7 ml for FEV1 and FVC, respectively, for each ppb increase in ozone (p < 0.001). Following correction for an individual's mean lung function level, date, and temperature, regression of either the afternoon or the daily change (afternoon-morning) corrected for the morning measurement of FEV1 and FVC on ozone showed similar magnitudes of effect. These associations were still apparent on the following morning, suggesting a persistent ozone effect. These results indicate that exposure of a population of outdoor workers to ambient ozone concentrations below 85 ppb is associated with decreased lung function over the day, which persists to the following day.