Background: Conjunctivitis has hazardous effects on patients' quality of life through influencing school performance, work productivity, and daily activities such as driving. However, limited evidence is available on the contributory role of air pollution on conjunctivitis, particularly in China. Methods: We obtained data of 81,351 conjunctivitis outpatients from the largest comprehensive hospitals of four cities, China, between Jan 1, 2013 and Dec 31, 2014. Data on air pollutants, including particulate matter <= 2.5 mu m in diameter (PM2.5), particulate matter <= 10 mu m in diameter (PM10), nitrogen dioxide (NO2), ozone (O-3) and sulphur dioxide (SO2) were collected from China National Environmental Monitoring Centre. Conjunctivitis outpatient visits were linked with air pollution concentrations by the visiting dates. A time-stratified case-crossover design with conditional logistic regression model was used to examine the effect of short-term exposure to air pollution on conjunctivitis outpatient visits. Results: We found that the associations between air pollutants (per 10 mu g/m(3) increase) and hospital outpatient visits for asthma were [odds ratios (ORs) and 95% confidence intervals]: 1.004(1.002-1.007) for PM2.5, 1.004 (1.002-1.005) for PM10, 1.012(1.005-1.020) for NO2, 1.006 (1.001-1.011) for SO2, and 1.007 (1.003-1.010) for O-3, respectively at lag0 day. Outpatients aged 35-64 years showed significant associations with exposure to PM2.5 (1.005, 1.001-1.010), PM10 (1.005, 1.002-1.008), NO2 (1.014, 1.003-1.026), and O-3 (1.005,1.000-1.011), while those aged 15-34 years showed significant associations with exposure to O-3 (1.010, 1.004-1.017). Conclusions: Short-term exposure to air pollution has impacts on outpatient visits for conjunctivitis in China. This study suggests that improving air quality could protect eye health. (C) 2019 Elsevier Ltd. All rights reserved.