Transgender men are routinely treated with testosterone (T). As androgen excess in cisgender women is associated with endothelial dysfunction, the effect of testosterone in transgender men is of considerable clinical importance. Flow-mediated vasodilation (FMD) was assessed in 11 transgender men receiving testosterone and 20 cisgender women during early follicular phase. Total T and free T were both greater in transgender men than in cisgender women (484.6 ± 122.5 versus 1.5 ± 0.7 ng/dl and 83.9 ± 32.4 versus 1.9 ± 0.8 pg/dl, respectively). FMD was lower in transgender men than in cisgender women (4.5 ± 2.7% versus 8.1 ± 2.9%, P = 0.002), indicating significantly poorer endothelial function in transgender men. As endothelial function is a marker of cardiovascular risk, it should be carefully monitored as part of the management of transgender men.