The aim of this prospective study was to classify cardiac involvement in myopathies by means of a comprehensive cardiac investigation, to determine the rate of cardiac involvement in myopathies according to this classification and to compare the validity of previously reported electro-cardiographic myopathy indices (QT/PQs, P/PQs, R/S) with that of the comprehensive cardiac investigation. We included 14 patients with myotonic dystrophy, 6 patients with Becker's muscular dystrophy and 10 patients with mitochondrial myopathy. Cardiac involvement was classified as either "definite", "equivocal" or "absent" by assessing cardiovascular history, physical examination, electrocardiography, echocardiography and 24-hour ambulatory electrocardiography. "Definite"/"equivocal"/"absent" cardiac involvement was found in 12/2/0 myotonic dystrophy, 3/3/0 Becker's muscular dystrophy and 6/3/1 mitochondrial myopathy patients. Electrocardiographic myopathy indices were pathologic in 3 Becker's muscular dystrophy, 6 mitochondrial myopathy but in none of the myotonic dystrophy patients. The proposed comprehensive cardiac investigation is simple, inexpensive and effective in assessing cardiac involvement in patients with myotonic dystrophy. Becker's muscular dystrophy and mitochondrial myopathy. In case of cardiac involvement, cardiac therapy might be one of the few therapeutic options for these patients.