AIMS: To evaluate the benefit of adding Losartan to baseline therapy in patients with Marfan syndrome (MFS). METHODS AND RESULTS: A double-blind, randomized, multi-centre, placebo-controlled, add on trial comparing Losartan (50 mg when \textless50 kg, 100 mg otherwise) vs. placebo in patients with MFS according to Ghent criteria, age \textgreater10 years old, and receiving standard therapy. 303 patients, mean age 29.9 years old, were randomized. The two groups were similar at baseline, 86% receiving β-blocker therapy. The median follow-up was 3.5 years. The evolution of aortic diameter at the level of the sinuses of Valsalva was not modified by the adjunction of Losartan, with a mean increase in aortic diameter at the level of the sinuses of Valsalva of 0.44 mm/year (s.e. = 0.07) (-0.043 z/year, s.e. = 0.04) in patients receiving Losartan and 0.51 mm/year (s.e. = 0.06) (-0.01 z/year, s.e. = 0.03) in those receiving placebo (P = 0.36 for the comparison on slopes in millimeter per year and P = 0.69 for the comparison on slopes on z-scores). Patients receiving Losartan had a slight but significant decrease in systolic and diastolic blood pressure throughout the study (5 mmHg). During the study period, aortic surgery was performed in 28 patients (15 Losartan, 13 placebo), death occurred in 3 patients [0 Losartan, 3 placebo, sudden death (1) suicide (1) oesophagus cancer (1)]. CONCLUSION: Losartan was able to decrease blood pressure in patients with MFS but not to limit aortic dilatation during a 3-year period in patients \textgreater10 years old. β-Blocker therapy alone should therefore remain the standard first line therapy in these patients