Background: The onset of acute myocardial infarction (AMI) varies substantially throughout the day, with bimodal peak in the morning and midnight hours. However, little is known about whether the cardiovascular (CV) outcome of AMI varies according to the time that the myocardial infarction (MI) occurs. We investigated the association between onset of chest pain and 6-month CV outcome in AMI. Methods: We investigated the circadian pattern and 6-month CV outcome of 469 consecutive patients (311 males; 67 짹 11 years) with AMI admitted to emergency room between November 2005 and December 2006. Results: We divided the day into 2-hour intervals. There were two significant peak times for onset of chest pain: midnight hour (midnight to 1:59 AM) and morning hour (8:00 AM to 9:59 AM). The 6-month major adverse cardiac event (MACE) including death, MI and revascularization was significantly higher (18.5% versus 32.7%, p=0.014) in midnight MI patients. Kaplan-Meier survival curve showed 6-month mortality was higher (9.9% versus 20.0%, p=0.027) in midnight MI patients. The number of patients with Killip class ��� II (p=0.009), taking diuretics (p=0.021), pro B-type natriuretic peptide (proBNP) (8175 pg/ml versus 3225 pg/ml, p=0.007) and high sensitivity C-reactive protein (hsCRP) (5.15 mg/dl versus 1.70 mg/dl, p=0.045) were higher in midnight MI patients. Hemoglobin (12.5 versus 13.2 g/dl, p=0.013) and cardiac troponin I (cTnI) (22.9 ng/ml versus 59.9 ng/ml, p=0.008) were lower. Multivariate analysis showed the nighttime onset of AMI was independent predictor of 6-month MACE (odds ratio [OR] 2.99, 95% confidence interval [CI] 1.06 to 8.46, p=0.039) and 6-month mortality (OR 2.72, 95% CI 1.21 to 6.09, p=0.015) after adjustment for age���65, sex, systolic blood pressure, white blood count, creatinine, proBNP, hsCRP, cTnI and total cholesterol. Conclusion: There were two significant peak times for onset of AMI. The 6-month MACE and mortality were significantly higher in midnight MI patients.