To investigate the acute respiratory health effects of ambient air pollution, the number of emergency of urgent daily respiratory admissions to 168 acute care hospitals in Ontario were related to estimates of exposure to ozone and sulfates in the vicinity of each hospital. Ozone levels were obtained from 22 monitoring stations maintained by the Ontario Ministry of the Environment for the period January 1, 1983 to December 31, 1988. Daily levels of sulfates were recorded at nine monitoring stations representing three different networks operated by the Ontario Ministry of the Environment and Environment Canada. Positive and statistically significant associations were found between hospital admissions and both ozone and sulfates recorded on the day of admission and up to 3 days prior to the date of admission. Five percent of daily respiratory admissions in the months of May to August were associated with ozone, with sulfates accounting for an additional 1% of these admissions. Ozone was a stronger predictor of admissions than sulfates. Positive and statistically significant (P < 0.05) associations were observed between the ozone-sulfate pollution mix and admissions for asthma, chronic obstructive pulmonary disease, and infections. Positive associations were also found in all age groups, with the largest impact on infants (15% of admissions associated with the ozone-sulfate pollution mix) and the least effects on the elderly (4%). Temperature had no effect on the air pollution-admission relationship. Ozone (lagged 1 day) and sulfates (lagged 1 day) displayed a positive association with respiratory admissions for 91 and 100% of the 168 acute care hospitals, respectively. Air pollution was not related to a class of nonrespiratory admissions, which served as a negative control, nor was it related to admissions in the winter months of December to March, when ozone and sulfate levels are low and when people spend a considerable amount of time indoors.