We utilized Doppler echocardiography to characterize left ventricular diastolic function in 42 patients with myotonic dystrophy (mean age 37 ± 12 years, 64% male) who had no symptoms of heart failure and had normal left ventricular systolic function. Data were compared with those in 41 normal control subjects of similar age and gender. Heart rate, systemic blood pressure, and cardiac dimensions (wall thickness, left atrial and left ventricular cavity dimensions) were similar and not significantly different in patients and controls. As a group, patients showed significantly increased deceleration time and decreased rate of decline of flow velocity in early diastole (p < 0.0001 and p < 0.01, respectively) when compared to controls. Individual patient analysis showed that 10 (24%) of the 42 patients with myotonic dystrophy had 2 or more abnormal Doppler indexes of diastolic function consistent with a pattern of impaired left ventricular relaxation. The most common abnormalities were increased deceleration time ( > 224 ms; 9 patients), prolonged isovolumic relaxation time ( > 103 ms; 8 patients) and reduced rate of decline of flow velocity in early diastole ( < 2.1 m/s²; 5 patients). In addition, peak early diastolic flow velocity was reduced ( < 43 cm/s) in 3 patients and early to atrial peak flow velocity ratio was reduced ( < 1) in 2 patients. Comparison of subgroups of patients with and without abnormal Doppler indexes showed no significant differences with regard to age, gender, heart rate, systemic blood pressure, severity of neuromuscular disease, and cardiac dimensions. After study, patients were clinically followed up for a mean period of 20 ± 7 months (range 12-35). During observation no patients died and none experienced symptoms of heart failure. This Doppler echocardiographic analysis demonstrates that diastolic abnormalities may be present in patients with myotonic dystrophy, even in the absence of symptoms of cardiac failure or left ventricular systolic dysfunction. These diastolic abnormalities suggest an intrinsic myocardial abnormality in patients with myotonic dystrophy; however, whether they represent a preclinical phase of myocardial involvement or an intrinsic feature of the primary myocardial disease process in myotonic dystrophy remains to be elucidated.