BACKGROUND Coronary slow flow (CSF) is characterised by delayed opacification of coronary arteries in the absence of epicardial occlusive disease. It has been reported that CSF may cause angina, myocardial ischaemia, and infarction. Fragmentation of QRS complex (fQRS) is an easily evaluated non-invasive electrocardiographic parameter. It has been associated with alternation of myocardial activation due to myocardial scar and/or ischaemia. Whether CSF is associated with fQRS is unknown. The presence of fQRS on ECG may be an indicator of myocardial damage in patients with CSF. OBJECTIVE To investigate the presence of fQRS in patients with CSF. METHODS Sixty patients (mean age 55.5 ± 10.5 years) with CSF and 44 patients with normal coronary arteries without associated CSF (mean age 53 ± 8.4 years) were included in this study. The fQRS was defined as the presence of an additional R wave or notching of R or S wave or the presence of fragmentation in two contiguous leads corresponding to a major coronary artery territory. RESULTS The presence of fQRS was higher in the CSF group than in the controls (p = 0.005). Hypertension was significantly more common in the CSF group (p < 0.001). There was no significant association between the presence of fQRS and an increasing number of vessel involvements. Logistic regression analysis demonstrated that the presence of CSF was the independent determinant of fQRS (OR = 10.848; 95% CI 2.385-49.347; p = 0.002). CONCLUSIONS Fragmented QRS, indicating increased risk for arrhythmias and cardiovascular mortality, was found to be significantly higher in patients with CSF. We have not found an association between the presence of fragmented QRS and the degree of CSF. Further prospective studies are needed to establish the significance as a possible new risk factor in patients with CSF.