Aims: This study examined clinical factors associated with sex differences in the use of acute reperfusion therapy (fibrinolysis or primary percutaneous coronary intervention) in ST-elevation myocardial infarction (STEMI) patients, and the interaction between sex and these factors in Sweden and Canada. Methods: Patients with STEMI in Sweden (n=32,676 from the Register of Information and Knowledge about Swedish Heart Intensive Care Admissions) were compared with similar patients in Canada (n=3375 from the Canadian Global Registry of Acute Coronary Events) for the period 2004–2008. Results: Unadjusted vs. age-adjusted odds ratios (OR) for no reperfusion (women vs. men) were for Sweden 1.57 (95% CI 1.49–1.64) vs. 1.14 (95% CI 1.08–1.20), and for Canada 1.61 (95% CI 1.39–1.87) vs. OR 1.18 (95% CI 1.01–1.39). Sex differences persisted after multivariable adjustments (including prehospital delay, atypical symptoms, diabetes), factors for which no interaction with sex was found. Among women