Objectives: The purpose of this study was to examine the associations between emergency department (ED) visits for conjunctivitis and ambient air pollution levels in urban regions across the province of Ontario, Canada. Material and Methods: Information from the National Ambulatory Care Reporting System was used to create time-series records, for the period of April 2004 to December 2011, on emergency department visits of patients suffering from conjunctivitis. A total of 77 439 emergency department visits for conjunctivitis were analyzed. A time-stratified case-crossover design was applied, completed with meta-analysis in order to pool inter-city results. Odds ratio (OR) for an emergency department visit was calculated in different population strata per one-unit increase (one interquartile range – IQR increase in a pollutant’s daily level) while controlling for the impacts of temperature and relative humidity. Results: Statistically significant positive results were observed in the female population sample, for nitrogen dioxide (NO₂) exposure lagged 5–8 days, with the highest result for the 7-day lag (OR = 1.035, 95% CI: 1.018–1.052) and for fine particulate matter with a median aerodynamic diameter of less than 2.5 μm (PM_2.5), for lags 6 and 7 days, with the highest result for lag 7 (OR = 1.017, 95% CI: 1.003–1.031). In the male population sample, statistically significant positive results were observed for NO₂ at lag 5 days (OR = 1.024, 95% CI: 1.004–1.045) and for ozone (O₃), at lags 0–3 and 7 days, with the highest result for lag 0 (OR = 1.038, 95% CI: 1.012–1.056). Also for males, statistically significant results were observed in the case of PM_2.5 exposure lagged by 5 days (OR = 1.003, 95% CI: 1.000–1.038) and sulfur dioxide (SO₂) exposure lagged by 1 and 2 days (OR = 1.016, 95% CI: 1.000–1.031 and OR = 1.018, 95% CI: 1.002–1.033). Conclusions: The findings of this study suggest that there are associations between levels of air pollution and ED visits for conjunctivitis, with different temporal trends and strength of association by age, sex, and season.