Abstract Introduction Despite several randomized trials, beneficial effects of Losartan and β-blockers in adults with Marfan syndrome (MFS) are not entirely clear. The COMPARE trial previously showed a small but significant beneficial effect of Losartan on top of β-blocker use on aortic root dilatation rate. Yet, this effect was not reproduced by other trials, although in general a favourable effect of Losartan could be demonstrated. All trials in adults with MFS showed lower than expected aortic root dilatation rates, suggesting mildly affected study populations. Therefore we extended the follow-up period of the COMPARE trial up to 10 years and aimed to assess the clinical outcomes of the study cohort, as well as the effect of medication regimes on aortic root dilatation rates in the subgroup of patients with a native aortic root at initial randomization. Methods Patients previously enrolled in the COMPARE trial were retrospectively analyzed. Cardiovascular events (aortic dissections, ruptures and operations) and all-cause mortality were assessed. Individual aortic root dilatation rates were estimated in patients with a native aortic root at time of randomization on the basis of linear regression analysis of multiple transthoracic echocardiogram (TTE) results. Correlations between aortic root dilatation rates and cumulative Losartan or β-blocker treatment days were assessed with Spearman's rho (ρ). Results During a median follow-up of 8.0 years, two dissections and three deaths occurred in the 151 patients with a native aortic root at time of randomization. The 122 patients that were eligible for aortic root dilatation analysis, underwent a median of 6 TTEs. The median aortic root dilatation rate in these patients was 0.28 (interquartile range 0.09 - 0.59) mm/y. These patients were further classified as either patients with a stable aortic root (n=102) or with aortic root growth (n=20), based on the threshold of 0.9 mm per year. Patients with aortic root growth had significantly more aortic root replacements during follow-up (17/20 vs 18/102, P=0.001). Furthermore, aortic root dilatation rate was negatively correlated with the number of Losartan treatment days (ρ=−0.272, P=0.003), β-blocker treatment days (ρ=−0.217, P=0.017) and with the duration of follow-up (ρ=−0.437, P<0.001). Conclusions Our results support previous findings that Losartan and β-blockers appear to be equally effective on aortic root dilatation rate in Marfan Syndrome patients. The low event rate in the long term follow-up of this subgroup of the COMPARE trial represent a relatively mildly affected study population and an aggressive prophylactic surgical regime.