['Metformin is one of the most commonly prescribed antihyperglycemic agents for the treatment of type 2 diabetes. However, little is known about the effect of metformin on no-reflow in diabetic patients.', 'In this study, we investigated retrospectively whether chronic pretreatment with metformin was associated with no-reflow in diabetic patients who underwent primary coronary intervention for acute myocardial infarction (AMI).', 'A total of 154 consecutive diabetic patients who underwent primary angioplasty for a first ST-segment elevation myocardial infarction were studied. No-reflow was defined as a final TIMI flow of ≤2 or final TIMI flow of 3 with a myocardial blush grade of <2. The no-reflow phenomenon was found in 53 of 154 patients. There were no significant differences in clinical characteristics between the patients with and without metformin pretreatment. However, the 65 patients receiving chronic metformin treatment before admission had lower incidence of the no-reflow than those without it (4.2 and 14.6%, P < 0.05). Multivariable logistic regression analysis revealed that absence of metformin pretreatment was a significant predictor of the no-reflow along with high-burden thrombus, ejection fraction on admission and anterior AMI.'] Is chronic pretreatment of metformin associated with the reduction of the no-reflow phenomenon in patients with diabetes mellitus after primary angioplasty for acute myocardial infarction?