Thirty-eight consecutive adult patients with myotonic dystrophy were included in a study with electrocardiography at rest and 24-hour ambulatory electrocardiography. The patients were subdivided into three groups according to the severity of the disease. The prevalence of abnormal electrocardiograms at rest was 31, 50 and 100% in patients with mild, moderate and severe disease, respectively. The main characteristics observed at ambulatory electrocardiography were a high frequency of sinus bradycardia (58%) and intermittent atrioventricular block II (8 %). These bradyarrhythmias were not correlated to the severity of the disease. Sustained atrial fibrillation or flutter was found in 3 patients (8%), all with the most severe form of the disease. Ambulatory electrocardiography should be used deliberately in the evaluation of patients with myotonic dystrophy and symptoms compatible with cardiac arrhythmias.