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BackgroundAcne can result from increased testosterone concentrations after gender-affirming masculinizing testosterone therapy (GATT) initiation.ObjectivesThe aim of this study was to determine the prevalence and risk factors of acne and acne severity in transgender individuals while receiving GATT.Materials & MethodsA prospective multicenter follow-up study (2010-2019) was performed to assess self-reported acne and acne severity, and define risk factors for acne in transgender men during the first three years after initiation of GATT (n=323). Investigated risk factors included: age at initiation of GATT, body mass index (BMI), type of testosterone administration, use of lynestrenol, alcohol use, smoking and serum testosterone concentrations during therapy.ResultsThe prevalence of moderate/severe acne increased from 11.8% to 39.1% after one year of GATT. Multivariate analyses showed BMI >25 kg/m2 (relative risk [RR]: 1.46; 95% confidence interval [CI]: 1.18-1.80), age: 18-25 years (RR: 1.98; 95% CI: 1.19-3.33), testosterone concentration >10 nmol/L (RR: 1.91; 95%CI: 1.28-2.84) and the presence of acne at baseline (RR: 1.82; 95%CI: 1.47-2.25) to be risk factors for development of moderate/severe acne.ConclusionAcne is a common side effect of GATT. Risk factors that influence the occurrence of moderate to severe acne in testosterone-treated transgender men are high BMI, younger age at initiation of GATT and testosterone concentrations within or above the target range. These observations could be taken into account when counselling transgender men starting GATT.