This study investigated the correlation between cardiac abnormalities and cytosine-thymine-guanine (CTG) trinucleotide repeat expansion in patients with myotonic dystrophy (MD). We studied 18 patients with the adult form of classical MD and 18 age-matched control subjects. In patients with MD, left ventricular systolic function at rest was not different from that in normal subjects. On the other hand, of Doppler parameters of diastolic function, mitral inflow peak early velocity and atrial velocity were significantly lower, and deceleration time and isovolumic relaxation time were significantly longer in patients with MD compared with normal controls. No significant correlation was observed between these diastolic parameters and CTG repeat expansion in the patients, although the parameters showed a positive correlation with age and neurologic symptom duration. Electrocardiographic conduction abnormalities were detected in 42% of patients. These patients showed a significantly longer symptom duration, although the size of CTG repeats was not different between the patients with and without conduction abnormalities. This study demonstrated that, in patients with MD, significant alterations in ventricular diastolic function (myocardial myotonia) occur in addition to conduction abnormalities. The size of CTG expansion is not a predictor of these cardiac involvements.