The purpose of the present study was to evaluate cardiac autonomic nervous system (ANS) in patients with myotonic dystrophy type 1 (DM1). The function of ANS was studied in 20 patients with DM1 and 15 healthy controls. All subjects were investigated by a battery of six cardiovascular autonomic tests and power spectral analysis of heart rate variability (HRV). Only one patient had normal autonomic function. Two (10%) patients had mild, 10 (50%) moderate and 7 (35%) severe autonomic dysfunction. Thirteen (65%) patients had vagal and 4 (20%) sympathetic hyperactivity. Seven (35%) patients had vagal and 15 (75%) sympathetic dysfunction. Eighteen (90%) patients had orthostatic hypotension. The 24-hour time domain parameters of SDNN (SD of the NN interval) and total power were significantly lower in DM1 patients than in healthy controls (p < 0.05). However, other parameters of HRV, such as SDANN (SD of the mean NN, 5-minute interval), low frequency (LF), high frequency (HF) power and the LF/HF ratio were somewhat lower in patients with DM1 than in controls, but this was not statistically significant. There was no significant relationship between autonomic dysfunction and the severity of the disease or CTG repeat length. There was also no correlation between HRV and age. Our findings suggest that sympathetic dysfunction and vagal predominance may both occur in patients with DM1.