The normal range for aortic root diameters employing the echocardiographic continuous recording technique was determined in 159 adult subjects without aortic valve disease or hypertension. In order to evaluate the accuracy of this noninvasive technique, the aortic root diameters as measured by ultrasound before operation in 31 patients with aortic valve disease were compared with their respective aortic annulus diameters, as determined at the time of valve replacement. The 159 normal subjects consisted of 78 men and 81 women whose ages ranged from 18 to 74 yr (average 38). The normal range of aortic root diameters in this group was 17 to 33 mm (mean 23.7). A significant difference (P<0.001) in aortic root diameters existed between men and women, which could not be explained by differences in body surface area. The mean diameter among the 81 women was 22.4±3.3 mm (I SD) compared with the mean diameter in the 78 men of 25±3.5 mm. These results indicate a normal range for aortic root diameters which is significantly lower than previously reported. The aortic root diameter which was measured preoperatively by ultrasound in the 31 patients with aortic valve disease averaged 25.1±1.9 mm and did not differ significantly from the actual aortic annulus diameter of 25.5±2.6 mm measured at the time of surgery. The aortic root measurements by echo were within 2 mm of the corresponding annulus diameter in 25 of the 31 operated patients (80%).