Abstract Objective We aimed to compare baseline characteristics, coronary angiogram findings, and in‐hospital outcomes between female and male patients with ST‐segment elevation myocardial infarction (STEMI) under the age of 45 years. Background Although sex differences in risk factor profile have been documented for young patients with STEMI, limited data exist on the prevalence of spontaneous coronary artery dissection in these patients. Methods As part of an ongoing hospital‐based registry of suspected STEMI, we analyzed the original data for 51 women under the age of 45 years matched with 93 men of similar age who underwent coronary angiography at two percutaneous coronary intervention centers, between January 2003 and December 2012. Two interventional cardiologists independently reviewed coronary angiograms for all patients. Results The mean age for all patients was 39 years (range, 24–44) and the overall prevalence of cigarette smoking, dyslipidemia, hypertension, and diabetes mellitus were 70, 32, 13, and 4%, respectively. Young women were more likely to present with spontaneous coronary artery dissection (22 vs. 3%, p = .003) and more of them experienced reinfarction during the hospital course (15 vs. 1%, p = .01). The in‐hospital mortality rate was 2% for both sexes. Conclusions Spontaneous coronary artery dissection is an important cause of myocardial infarction in young female adults, accounting for 22% (95% confidence interval, 11–35%) of women with STEMI under the age of 45 years. The true prevalence of spontaneous coronary artery dissection might even be underestimated, because of the limited availability of advanced imaging techniques at the time of our study.