In this study, we examine the impact of ozone on daily respiratory admissions in both young children and the elderly in greater Vancouver, British Columbia. Study subjects included children less than 3 yr of age and adults 65 yr of age or over living in greater Vancouver who had acute hospital admissions for any respiratory diseases (ICD-9 codes 460-519) during the 13-yr period 1986-1998. Bidirectional case-crossover analysis was used to investigate associations between ambient ozone and respiratory hospitalizations after adjustment for other pollutants, including carbon monoxide, nitrogen dioxide, sulfur dioxide, and coefficient of haze. Potential effect modification by socioeconomic status as measured by household income was also examined. Respiratory admissions were associated with ozone levels 2, 3, 4, and 5 days prior to admission in both children and the elderly, with the strongest association observed at a lag of 4 days. Odds ratios for hospital admission of 1.22 (95% CI: 1.15-1.30) for children and 1.13 (1.09-1.18) for the elderly, respectively, were found, based on an increment in exposure corresponding to the interquartile range for ozone. Adjusting for other pollutants did not attenuate the ozone effect on respiratory admissions. Nor did socioeconomic status appear to modify the association between ozone and respiratory admissions in either children or the elderly. We concluded that ambient ozone is positively associated with respiratory hospital admission among young children and the elderly in Vancouver, British Columbia. These associations persisted after adjustment for both copollutant exposures and socioeconomic status.