The recent report by Pees et al suggests that losartan decreases normalized aortic dimensions in patients with the Marfan syndrome. The authors do not take into account the normal decrease in the aortic diameter/body surface area (BSA) ratio, which occurs during childhood. They use the ratio as if it were constant, when in fact cardiovascular diameters are not linearly related to BSA. For example, based on a large study of normal children, the average aortic diameter at the sinotubular junction of a 0.7-m 2 4-year-old is 14 mm, giving an aortic diameter/BSA “index” of 20 mm/m 2 . The corresponding index for a 1.4-m 2 adolescent is 12.8 mm/m 2 (18 mm/1.4 m 2 ). If the subject had been treated with losartan, it might be (wrongly) concluded that the drug was responsible for the lower ratio. The changes in aortic diameter/BSA in the authors’ Figures 1 and 2 could simply be the natural change in the ratio with time. Because the authors do not include the changes in the ratio for the control subjects, comparison is not possible. The hazards of attempting to “index” biologic measurements to BSA have been recognized for over 60 years. The use of z scores based on normal values is a better way to measure changes in cardiovascular dimensions, which occur during growth.