Bicuspid aortic valve (BAV) aortopathy is defined by dilation of the aortic root (AoRt) and/or ascending aorta (AsAo), and increases risk for aortic aneurysm and dissection. The effects of medical prophylaxis on aortic growth rates in moderate to severe bicuspid aortopathy have not yet been evaluated. This was a single-center retrospective study of young patients (1 day - 29 years) with bicuspid aortopathy (AoRt or AsAo z-score ≥ 4 SD, or absolute dimension ≥ 4 cm), treated with either losartan or atenolol. Maximal diameters and BSA-adjusted z-scores obtained from serial echocardiograms were utilized in a mixed linear effects regression model. The primary outcome was the annual rate of change in AoRt and AsAo z-scores during treatment, compared with before treatment. The mean ages (years) at treatment initiation were 14.2±5.1 (losartan; n = 27) and 15.2±4.9 (atenolol; n = 18). Median treatment duration (years) was 3.1 (IQR 2.4, 6.0) for losartan, and 3.7 (IQR 1.4, 6.6) for atenolol. Treatment was associated with decreases in AoRt and AsAo z-scores (SD/year), for both losartan and atenolol (pre- vs. post-treatment): losartan/AoRt: +0.06±0.02 vs. -0.14±0.03, p < 0.001; losartan/AsAo: +0.20±0.03 vs. -0.09±0.05, p < 0.001; atenolol/AoRt: +0.07±0.03 vs. -0.02±0.04, p = 0.04; atenolol/AsAo: +0.21±0.04 vs. -0.06±0.06, p < 0.001. Treatment was also associated with decreases in absolute growth rates (cm/year) for all comparisons (p ≤ 0.02). Medical prophylaxis reduced proximal aortic growth rates in young patients with at least moderate and progressive bicuspid aortopathy.