['Myocardial fibrosis is a well-known side effect of radiotherapy. Fragmented QRS (fQRS) has been shown to be a marker of myocardial fibrosis. We postulated that radiotherapy induces development of fQRS in breast cancer patients.', 'Breast cancer patients receiving locoregional radiotherapy were enrolled. Patients who had fQRS on electrocardiography (ECG) before radiotherapy were excluded. All patients were revaluated for the development of fQRS at 1-year follow-up. An age-matched healthy group served as controls.', 'A total of 52 breast cancer patients receiving locoregional radiotherapy were included (median age 49 years, interquartile range (IQR) 13). Of these, 19 (37%) had developed fQRS at 1-year follow-up. Compared with the control group, prevalence of fQRS was significantly higher in the irradiated group (37 vs. 12%; p < 0.002). Median total cardiac radiation dose was significantly higher in patients who had developed fQRS (5 Gy, IQR 5.2 vs. 1.7 Gy, IQR 4.4; p = 0.003). Cardiac radiation dose, entered either as a continuous variable (odds ratio (OR) 1.35, 95% confidence interval (CI) 104-1.74) or as a dichotomized variable (≥ 2.2 Gy, OR 6.48, 95% CI 1.47-28.61), was independently associated with the development of fQRS at 1-year follow-up.'] Does radiotherapy induce Development of Fragmented QRS in Patients with Breast Cancer?