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Background: Inconsistent findings have been reported regarding the effect of ambient temperature on ischemic stroke. Furthermore, little is known about how comorbidities and low socioeconomic status influence the association. We investigated the relationship between temperature and emergency department (ED) visits for ischemic stroke, and aimed to identify susceptible populations.Methods: Using medical claims data, we identified ED visits for ischemic stroke during 2005-2009 in Seoul, the Republic of Korea. We conducted piecewise linear regression analyses to find optimum temperature thresholds in summer and winter, and estimated the relative risks (RR) and 95% confidence intervals (CI) per 1°C change in temperature above/below the thresholds, adjusting for relative humidity, holidays, day of the week, and PM10 and O3levels.Results: There were 63,564 ED visits for ischemic stroke during the study period. In summer, the risk of ED visits for ischemic stroke was not significant, with the threshold at 26.8°C. The RR of ischemic stroke visits was 1.055 (95% CI, 1.006-1.106) above 25.0°C in medical aid beneficiaries and 1.044 (1.007-1.082) above 25.8°C in patients with diabetes. In winter, the risk of ischemic stroke significantly increased as the temperature dropped below the threshold. The RRs were 0.618 (95% CI, 0.453-0.842) and 0.643 (0.501-0.824) for hypertensive and patients with diabetes, respectively.Conclusions: Temperature increases above a threshold were positively associated with ED visits for ischemic stroke in patients with diabetes and medical aid beneficiaries in summer. In winter, a decrease in temperature, to a point, significantly increased the risk of ischemic stroke visits.