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Backgrounds/aims Any association of secondhand smoke (SHS) with refractive astigmatism (RA) and corneal astigmatism (CA) remains unknown. This study aims to investigate the associations of SHS exposure with RA and CA in schoolchildren.Methods We conducted a population-based, cross-sectional study involving 11 545 children aged 6–8 years from the Hong Kong Children Eye Study. SHS exposure was evaluated through questionnaires, quantified by parental smoking. Child astigmatism was defined by cycloplegic autorefraction and keratometry. The associations of SHS exposure with risks and magnitudes of RA and CA were tested using logistic and linear regressions.Results 26.47% of children were exposed to SHS (mean age±SD, 7.35±0.88 years; 51.1% boys). SHS exposure was associated with the risk (OR (95% CI), 1.11 (1.00 to 1.23), p=0.04) and magnitude of RA (β (SE), −0.03D (0.01), p=0.046). Exposure to ≥10 cigarettes/day was associated with risk (OR (95% CI), 1.64 (1.23 to 2.17), p<0.001) and magnitude of RA (β (SE), −0.11D (0.05), p=0.01). Exposure to ≥20 cigarettes/day was associated with RA risk (OR (95% CI), 2.18 (1.48 to 3.19), p<0.001), RA magnitude (β (SE), −0.20D (0.06), p=0.001) and CA magnitude (β (SE), 0.17D (0.070, p=0.01). Test for linear trend with SHS quantity was significant for both risk (p=0.004) and magnitude of RA (p=0.03).Conclusion SHS exposure increases the risk and severity of child RA dose-dependently. Reduction of smoking in the presence of children or better provision of a smoke-free living environment for children is recommended.