Women with Turner's syndrome (TS) have a threefold increase in mortality, primarily as a result of their cardiovascular complications. Recently, the risk of fatal aortic dissection has come to light as a major cause of mortality in women with TS. The aim of this study was to assess the prevalence of aortic root dilatation in a group of women with TS and to investigate the factors contributing to its development. Thirty-eight women with TS attending a dedicated adult Turner clinic were examined clinically and by M-mode and two-dimensional echocardiography on at least one occasion. Aortic root dilatation was defined as an aortic root diameter greater than the 95th centile for body surface area. Fasting serum lipid concentrations were measured in all women. Additionally, 18 subjects underwent noninvasive assessment of central arterial stiffness using applanation tonometry. Fifty percent of subjects were hypertensive and a similar number had an abnormal echocardiogram. A bicuspid aortic valve was present in 33% of subjects, 16 women (42%) had ascending aortic root dilatation. This was associated with a bicuspid aortic valve in four women and hypertension in 11. Two women had isolated aortic root dilatation. Aortic root diameter was significantly associated with systolic blood pressure (r = 0.5, P = 0.003) and left ventricular thickness (r = 0.5, P = 0.02). There was no association with serum lipids or arterial compliance. Structural cardiac abnormalities are present in up to 50% of women with Turner's syndrome. Aortic root dilatation is a significant risk in women with Turner's syndrome and is closely dependent on blood pressure. Aortic root dilatation does not appear to be related to atherosclerosis and is more likely to be due to a mesenchymal defect. Regular surveillance of the aortic root diameter is essential in all women with Turner's syndrome and hypertension should be treated aggressively when present in order to minimize the risk of potentially fatal aortic dissection.