The leading cause of premature death in patients with Marfan's syndrome (MS) is type A aortic dissection or rupture due to progressive aortic root dilation. The aim of this study was to analyze aortic root growth in 113 men and 108 women with MS. All patients were prospectively followed with serial echocardiograms of the native aortic root. At baseline, women had on average a 5-mm smaller aortic root diameter adjusted for age than men. Average aortic root growth was 0.42 mm/year (SE 0.05) in men and 0.38 mm/year (SE 0.04) in women. On the basis of aortic root growth rates, the men and women could be divided into 2 normally distributed subgroups: fast and slow growers. Approximately 1 in 7 men (1.5 mm/year, SE 0.5) and approximately 1 in 9 women (1.8 mm/year, SE 0.3) had fast-growing aortic root diameters. Significantly more type A dissections (25% vs 4%, p <0.001) were observed in fast growers than in slow growers; this was found in men and women. Type A dissections were observed in 4 men and 9 women. By reducing the cut-off value by 5 mm for elective aortic root replacement in women, type A dissections could have been prevented in 3 women. In conclusion, guidelines should take gender differences into account, and therefore, the investigators propose reducing the threshold for elective aortic root replacement in women with MS by 5 mm.