We hypothesize that higher morbidity of patients with ST-segment elevation myocardial infarction (STEMI) in the out-of-office hours differences in outcome after myocardial infarction may depend on the concentrations of inflammatory cytokines. The aim of the study was to determine the relation between the time of percutaneous coronary intervention (PCI) and local concentration of interleukin 6 (IL-6) and its soluble receptors (sIL-6R and sgp130) in patients with STEMI. Methods and results: The study included 32 patients with invasively treated left anterior descending artery occlusion and no significant co-morbidities. Blood samples were drawn from coronary sinus and aorta before and after intervention. Patients admitted in the afternoon (13–20) presented significantly higher mean IL-6 levels in all samples than patients admitted in the morning. There was a positive correlation between time of intervention and concentrations of IL-6 in all samplings, but also with transcardiac IL-6 gradient at the end of procedure and IL-6 increase during PCI. We did not find any significant association between time of PCI and concentrations of sIL-6R and sgp130, time from pain to balloon, angiographic parameters or medical history. Conclusions: Coronary concentration of IL-6 in patients with STEMI is significantly higher in the afternoon than in the morning. This might be involved in increased morbidity of those patients.