The relationship regarding time of percutaneous coronary intervention (PCI) and clinical outcomes in patients with acute myocardial infarction (AMI) treated within the left main coronary artery (LMCA) is less investigated compared to the overall group of patientswith AMI. Therefore, we aimed to assess the relationship between time of PCI (day- vs night-time) and overall mortality rate in patients treated due to AMI within the LMCA. This cross-sectional study included 443,805 AMI patients hospitalized between 2006 and 2018 enrolled in the Polish Registry of Acute Coronary Syndromes. We extracted 5,404 patients treated within the LMCA. The number of patients were treated during daytime hours (7:00 am-10:59 pm) was 2809 while 473 patients underwent treatment during night-time hours (11:00 pm-6:59 am). Differences in cardiac mortality rates between night- and day-hours among patients treated with PCI during the follow-up period were assessed via the Kaplan-Meier method. The 30-day (20.3% vs 14.9%, P = .003) and 12-month (31.7% vs 26.2%, P=.001) overall mortality rates were significantly greater among patients treated during night-time, which was confirmed by comparison using Kaplan-Maier survival curves (P=.001). The time of PCI was not found among predictors of survival in multiple regression analysis (hazard ratio: 1.22; 95% confidence interval: 0.96-1.55, P=.099). Patients treated during night-time in comparison to the day-time are related to higher in-hospital, 30-day and 12-month mortality. This is probably largely a consequence that the night-time, in comparison to the day-time, of treatment of patients with AMI with PCI within the LMCA is and indicator of higher comorbidity and clinical acuity of patients undergoing therapy. Therefore, the night-time was not found to be an independent predictor of greater mortality rate during the 12-months follow-up period.