The objective of this study was to determine if calculation of the sinus of Valsalva to annulus ratio (SOV/Ann) effectively screens for root dilatation (ARD) during two-dimensional (2D) echocardiography in young patients (1 month-15 years) with suspected or confirmed Marfan syndrome. The aortic root was imaged using 2D echocardiography (parasternal long-axis view) and the SOV/Ann ratio calculated/analyzed in 27 Marfan patients and 45 age-matched controls. All controls had a SOV/Ann ratio <1.45, and 82% of Marfan's with ARD had a SOV/Ann ratio 1.45. Five of the Marfan patients with ARD had a SOV/Ann ratio < 1.45 due to normalization of the SOV/Ann ratio because of pathologic annular enlargement. Determination of the SOV/Ann ratio allows rapid on-line screening for aortic root dilatation during 2D echocardiography. A SOV/Ann ratio 1.45 predicted ARD in young Marfan patients with a high degree of accuracy (sensitivity = 0.82, specificity = 1.00). Screening for ARD using the SOV/Ann ratio fails when annular dilatation has progressed to the degree that the SOV/Ann ratio normalizes (SOV/Ann < 1.45). In this subgroup of patients, determination of aortic root dilatation will have to be evaluated by standard methods.