Since 2008, when angiotensin II type I receptor blockade with losartan was introduced in the prevention of cardiovascular manifestation of Marfan syndrome (MFS), a specific treatment to address the cardiovascular lesions became available. The present study aimed to compare the response of such in an unselected cohort of patients with genotyped MFS. At a tertiary university children's hospital, 20 pediatric and adolescent patients aged 1.7 to 21.6 years with genetically proven MFS were enrolled in a prospective treatment study of losartan for evaluation of the aortic dimensions and elasticity indexes. The mean follow-up period was 33 ± 11 months. A significant reduction in the normalized aortic dimensions with losartan was observed in the valve, root, sinotubular junction, and ascending aortic segments (p = 0.008, p <0.001, p = 0.012, and p = 0.001, respectively). No correlation between elasticity behavior and the decrease in the aortic dimension with losartan therapy was detectable. A significant correlation between stronger improvement and younger age at onset (r = 0.643, p = 0.002) and a longer therapy duration (r = −0.532, p = 0.016) was verifiable. However, no correlation between improvement with therapy and the type of mutation or presentation of clinical forms was remarkable. Elasticity also seemed to improve but not significantly. In conclusion, in our cohort of young patients with MFS, a significant improvement with losartan monotherapy was proved in all affected proximal aortic segments, with a better response to therapy when started at an earlier age and with a longer therapy duration.