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Background: Pre-hospital delays have been associated with poor outcomes in patients with acute ST-elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PCI). It is currently unknown how environmental variables affect treatment delays in these patients., Methods and Results: The association between environmental variables, time to treatment including transportation times and adverse in-hospital events was assessed in 1828 consecutive patients with STEMI undergoing primary PCI between 2010 and 2014 in the Montreal metropolitan area. Median[Q1;Q3] total ischemia time was significantly longer during summer season (April-September) as compared to winter season (October-March, 201[140;305] min vs 187[126;266] min, p = 0.022). This difference between seasons was due to a significant increase in median decision time to seek treatment for symptoms during summer (90[46;185] min vs 78[40;156], p = 0.004). The former peaked during July and August and was most pronounced in men. Hence, outside temperature and summer season were identified as strong predictors of prolonged decision time in patients with STEMI (p < 0.001 and p = 0.002, respectively). Transportation times slightly increased during winter season and snow fall, this difference, however, was not significant (p = 0.46). A significant increase in in-hospital adverse outcomes following primary PCI was observed during summer season as compared to winter season (7.2% vs 4.8%, p = 0.032). Accordingly, multivariate logistic regression models adjusted for baseline variables identified summer season as a strong predictor of periprocedural adverse events (OR 1.83, 95% CI 1.2-3.11, p = 0.037)., Conclusion: Contrary to our initial hypothesis, pre-hospital delays in patients with STEMI are considerably longer and associated with adverse in-hospital outcomes during summer season. Considering the consequences of global warming, it is imperative that educational efforts targeting patients' perception are implemented to counter treatment delays., (Copyright © 2018 Elsevier B.V. All rights reserved.)