Madsen MC, van Dijk D, Wiepjes CM, et al. - Testosterone therapy is known to induce erythrocytosis that can increase the risk of thromboembolic events. Researchers here examined the prevalence of erythrocytosis among trans men using testosterone therapy as well as investigated determinants in its development. They performed a 20 year follow-up study in adult trans men who initiated testosterone therapy, and had monitoring of hematocrit at their center (n = 1,073). Occurrence of erythrocytosis was observed in 11% (hematocrit > 0.50 l/l), 3.7% (hematocrit > 0.52 l/l) and 0.5% (hematocrit > 0.54 l/l) of trans men suggesting erythrocytosis occurrence in trans men using testosterone. Researchers identified the largest increase in hematocrit in the first year, but also after the first years, a substantial number of people were identified presenting with hematocrit > 0.50 l/l. In the care for trans men with erythrocytosis while on testosterone therapy, an important initial step is to advise them to quit smoking and to switch to a transdermal administration route and if BMI is high, to lose weight.