AIMS: To determine the effect of intravenous administration of nicorandil on slow coronary flow (SCF) phenomenon in patients undergoing percutaneous coronary intervention (PCI). METHODS AND RESULTS: In a preliminary study, 6 mg of nicorandil showed optimal efficacy for vasodilatation without causing significant haemodynamic instability. In the main study, a total of 408 patients were randomly assigned to receive intravenous administration of 6 mg of nicorandil immediately before PCI. The number of patients in the nicorandil group was 206 [acute coronary syndrome (ACS): 47, non-ACS: 159] and that in the control group was 202 (ACS: 61, non-ACS: 141). Nicorandil significantly decreased the incidence of post-procedural SCF phenomenon in both the ACS and non-ACS groups. The rate of target vessel revascularization (TVR) was significantly lower in the nicorandil group than in the control group in ACS patients. CONCLUSION: Our simple procedure prevented SCF phenomenon not only in patients with ACS but also in patients with non-ACS without any adverse effect. Additionally our procedure reduced the rate of TVR in patients with ACS.