Background: Data on the efficacy of coronary angioplasty in patients with ST-segment elevation myocardial infarction (STEMI) treated during off-shift hours are limited, but some publications suggest a worse outcome in this group of patients. Aim: To compare the results of percutaneous coronary interventions (PCI) in STEMI patients admitted to hospital during the daytime and off-shift hours and to identify factors which influence prognosis. Methods: From January 1998 to October 2003, 1992 patients with STEMI were hospitalised and 1778 of them were treated with immediate PCI, including 482 admitted in the daytime (weekdays 8 a.m. – 3 p.m.; group I) and 1296 during off-shift hours (weekdays 3 p.m. – 8 a.m., weekends and holidays; group II). The clinical characteristics of both groups were similar, except for less frequent hypercholesterolaemia in the daytime group (52.1 vs. 59.7%; p=0.0041). Results: There was no significant difference between the groups regarding time from admission to angiography (30 vs. 25 minutes), rates of reocclusion (5.2 vs. 4.9%), stroke (1.4 vs. 1.6%), haemorrhagic complications (8.1 vs. 6.9%), in-hospital mortality (6.8 vs. 6.2%) and long-term (24 months) mortality (13.7 vs. 13.6%) (p >0.05 for all parameters). The rate of stent implantation was significantly higher in the daytime group (71.2 vs. 66.2%; p=0.047). Conclusions: The proper organisation of duties of the division of cardiology and the cardiac catheterisation laboratory, with focus on providing onsite staffing of the cardiac catheterisation laboratory around the clock, enables similar results of PCI in patients with ST-segment elevation myocardial infarction treated during off-shift hours as compared with patients treated during the daytime.