Objective: The risk of aortic dissection is 100 fold increased in Turner syndrome (TS). Increased blood pressure (BP) and heart rate is present as well as an increased risk of ischemic heart disease and diabetes. This study aimed to prospectively assess heart rate variability (HRV) in TS and its relation to aortic dimensions. Methods: Adults with TS (n=91, aged 37.4±10.4 years) recruited through the Danish National Society of Turner Syndrome Contact Group and an endocrine outpatient clinic were examined thrice (mean follow-up of 4.7±0.5 years). Healthy controls (n=64, aged 39.4±12.1 years) were examined once. Aortic dimensions were measured at nine positions using 3D, non-contrast and free-breathing cardiovascular-MRI. HRV measured by short-term spectral analysis (supine-standing), transthoracic echocardiography, 24-hour ambulatory BP were done. Results: The changes in High frequency (HF) power (vagal activity) and Low-frequency:High-frequency-ratio (sympatho-vagal balance) was diminished in TS compared with controls when assessed by a two-way analysis of variance for the interaction term “Position (supine-standing) * status (TS or control)” (p<0.001). HF was lower while supine (p=0.001) and higher while standing (p=0.09) in TS compared to controls. Aortic diameter was inversely correlated with LF (r-average=-0.337 and -0.334, supine and standing; p<0.05) and HF (r-average=-0.405 and -0.293, supine and standing; p<0.05) in controls. Same degree of correlation was present in TS: LF (r-average=-0.312 and -0.341; p<0.05) and HF (r-average=-0.330 and -0.307; p<0.05). Changes in aortic diameter did not correlate with any measures of HRV. Prospectively there were no changes in HRV. Conclusions: A perturbed sympatho-vagal balance is present in TS explained by a decreased vagal activity in the supine position and increased vagal activity in the standing position. LF and HF correlate with aortic diameter in both groups, however no relation was found with changes in aortic diameter.