The effects of tropospheric ozone on lung function and respiratory symptoms have been well documented at relatively high concentrations. However, previous investigations have failed to establish a clear association between tropospheric ozone and respiratory diseases severe enough to require hospitalization after controlling for climate, and with gaseous and particulate air pollution at the lower concentrations typically observed in Canada today. To determine if low levels of tropospheric ozone contribute to hospitalization for respiratory disease, air pollution data were compared to hospital admissions for 16 cities across Canada representing 12.6 million people. During the 3927-day period from April 1, 1981, to December 31, 1991, there were 720,519 admissions for which the principle diagnosis was a respiratory disease. After controlling for sulfur dioxide, nitrogen dioxide, carbon monoxide, soiling index, and dew point temperature, the daily high hour concentration of ozone recorded 1 day previous to the date of admission was positively associated with respiratory admissions in the April to December period but not in the winter months. The relative risk for a 30 ppb increase in ozone varied from 1.043 (P < 0.0001) to 1.024 (P = 0.0258) depending on the selection of covariates in the regression model and subset of cities examined. The association between ozone and respiratory hospitalizations varied among cities, with relative risks ranging from 1.000 to 1.088 after simultaneous covariate adjustment. Particulate matter and carbon monoxide were also positively associated with respiratory hospitalizations. These results suggest that ambient air pollution at the relatively low concentrations observed in this study, including tropospheric ozone, is associated with excess admissions to hospital for respiratory diseases in populations experiencing diverse climates and air pollution profiles.