Autonomic dysfunction has been already described in patients with SCA3/MJD, but several important questions remain unanswered. The objectives of this study are to determine the frequency and the intensity of autonomic manifestations in SCA3/MJD, as well as to identify possible correlations between autonomic manifestations and genetic and clinical parameters. We have performed clinical and electrophysiological evaluations of 40 patients with SCA3/MJD and 38 healthy controls. We used the Scale for the Assessment and Rating of Ataxia (SARA) and the scales for Outcomes in Parkinson's Disease: Autonomic Questionnaire to quantify the severity of ataxia and autonomic complaints, respectively. We also studied heart rate variability at rest, during orthostatic challenge (30:15 ratio), Valsalva maneuver (Valsalva index), and deep breathing (E/I ratio). We evaluated spectral analyses of RR intervals at rest and the sympathetic skin response. Mean RR intervals at rest and the 30:15 ratio were different between patients and controls (811.8 versus 933.4 ms; p = 0.001 and 1.10 versus 1.15; p = 0.038, respectively). The Valsalva index and the E/I ratio were similar between the groups (p = 0.373 and p = 0.08). Spectral analysis presented distinct results in patients and controls, related to low- and high-frequency power (p < 0.001 and <0.001, respectively). We found cardiovascular and sympathetic sweat disautonomia in 30 % and 45 % of the patients with SCA3/MJD. Autonomic manifestations were related neither to genetic (CAG repeat length) nor clinical parameters (age, disease duration, SARA scores). Autonomic dysfunction is frequent and sometimes disabling in SCA3/MJD. We found evidence of both cardiovascular and sudomotor dysfunction in the disease.12513-9