Circadian variation is known as an important factor in acute myocardial infarction (AMI). Moreover, the circadian pattern may help in disease prevention and better medication prescription. Therefore, the aim of our study was to investigate the circadian pattern of symptom onset in patients with ST-segment elevation myocardial infarction (STEMI). This cross-sectional study was conducted on 777 patients admitted to the Imam Ali Cardiovascular Center, Kermanshah, Iran, with a diagnosis of STEMI from March 2018 to February 2019. Data were collected using a checklist developed based on the study's objectives. Differences between subgroups were assessed using one-way analysis of variance (ANOVA) with post-hoc testing and chi-square test (or Fisher's exact test). Out of the 777 patients, 616 (79.3%) were men. The mean and standard deviation (SD) of age of the patients was 60.93 ± 12.86 years. 380 patients (48.9%) were current smoker, 40.3% were hypertensive, 21.1% had hypercholesterolemia, 18.3% had diabetes mellitus (DM), 25.2% had history of angina, and about 15.0% had history of myocardial infarction (MI). The occurrence of STEMI was most common during hours between 06:01-12:00 (27.7%), followed by 12:01-18:00 (27.3%), 00:00-06:00 (24.3%), and 18:01-24:00 (20.7%), respectively. Gender was significantly associated with circadian pattern of STEMI. Women showed a double peak of symptom onset in 06:01-12:00 and 12:01-18:00. The present study of Iranian patients displayed circadian pattern of STEMI with 2 peaks in the morning and afternoon, and the both peaks were dominated by women. © 2020 Isfahan Cardiovascular Research Center & Isfahan University of Medical Sciences.