We analyzed by means of autoregressive spectral analysis the spontaneous beat-to-beat heart-rate variability (HRV) of 10 myotonic dystrophy (MD) patients (4 men and 6 women, aged 37-53 years) and 10 age- and sex-matched healthy, sedentary humans (control) at rest in the supine position. All MD patients had no cardiac conduction disturbances (i.e., atrioventricular or intraventricular conduction defects) on 12-lead electrocardiogram and were able to walk and perform daily activities. In the MD group, the total power, the power of the low-frequency component (a marker of sympathetic and vagal modulation of heart rate) and that of the high-frequency component (a marker of vagal modulation of heart rate) were smaller than those in the control group (P < 0.01, P < 0.05 and P < 0.05, respectively). The results of this study suggest that the cardiovascular autonomic nervous system contributing to the HRV may be disturbed even in the MD patients who can walk and perform daily activities. Therefore, one must give careful consideration to the cardiovascular autonomic dysfunction, as well as the cardiac conduction disturbance in the MD patients.