Background: Patients with Marfan syndrome may develop aortic root dissection despite only mild aortic root dilation as shown by standard echocardiography, which may be due to aortic root asymmetry. Purpose of the present study was to investigate aortic root asymmetry by magnetic resonance (MR) imaging in patients with Marfan syndrome and to compare these measurements with standardly performed echocardiography. Methods: Eighty-seven Marfan patients (mean age 31 ± 8 years) underwent MR imaging. From this population, 15 patients (mean age 29 ± 3 years) were selected in whom both echocardiography and MR imaging had been performed within 3 months. With echocardiography, the aortic root was measured according to the recommendations of the American Society of Echocardiography. With MR imaging, a short axis view of the aortic root was obtained to measure distances between the noncoronary, right coronary and left coronary cusps and the aortic root area. Correlations between aortic root area and diameters were assessed, and 95% confidence intervals (95% CIs) calculated. Results: No difference in the standardly measured noncoronary to right coronary cusp diameter between MR imaging and echocardiography was shown (42 ± 6 mm). Largest aortic root diameter on the MR images was the right to left coronary cusp diameter (46 ± 7 mm, p < 0.02) . For a given noncoronary to right coronary cusp diameter, 95% confidence intervals revealed a variation of -20 to +20% in the aortic root area. Conclusions: The majority of Marfan patients show asymmetric dilation of the aortic root by MR imaging. This phenomenon may go unnoticed when standard echocardiography is performed. The asymmetry of the aortic root might be of clinical importance in unexpected aortic root dissection.