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The majority of patients with Duchenne muscular dystrophy (DMD) have electrocardiographic abnormalities, but the clinical significance of conduction disturbances remains unclear. This study aimed to evaluate age-dependent cardiac conduction disturbances by electrocardiogram (ECG) to identify risks of complete atrioventricular (AV) block in this patient population.In total, 47 patients with DMD (age, ≥20 ys) who recorded ECGs at our hospital from July 2015 to June 2016 were included in this study. The PR interval and QRS duration in their previous ECGs were analyzed retrospectively. Associations between ECG findings and left ventricular (LV) systolic function obtained from the latest echocardiography were examined.The mean age of patients was 27.6 ± 6.0 years, and the mean observation period was 9.8 ± 3.7 years. The PR interval gradually increased with age, but no ECGs showed an abnormally prolonged PR interval. On the other hand, the QRS duration tended to increase progressively with age, and some patients had an abnormally prolonged QRS duration. The QRS duration was not correlated with LV systolic function (P = 0.867). One patient who developed a complete AV block had a drastically prolonged QRS duration before the onset of the disorder.The QRS duration tended to increase progressively with age, irrespective of LV systolic function in patients with DMD. Attention should be paid to the QRS duration as an indicator of risk for complete AV block in older patients.