Introduction: Current treatment guidelines for adolescents with gender dysphoria recommend therapy with gonadotropin-releasing hormone agonists (GnRHa) and testosterone in transgender males. However, most evidence on the safety and efficacy of testosterone is based on studies in adults. Aim: This study aimed to investigate the efficacy and safety of testosterone treatment in transgender adolescents. Methods: The study included 62 adolescents diagnosed with gender dysphoria who had started GnRHa treatment and had subsequently received testosterone treatment for more than 6 months. Main Outcome Measure: Virilization, anthropometry, laboratory parameters, and bone mineral density (BMD) were analyzed. Results: Adolescents were treated with testosterone for a median duration of 12 months. Voice deepening began within 3 months in 85% of adolescents. Increased hair growth was first reported on the extremities, followed by an increase of facial hair. Acne was most prevalent between 6 and 12 months of testosterone therapy. Most adolescents had already completed linear growth; body mass index and systolic blood pressure increased but diastolic blood pressure did not change. High-density lipoprotein (HDL) cholesterol and sex hormone binding globulin significantly decreased, but hematocrit, hemoglobin, prolactin, androstenedione, and dehydroepiandrosterone sulfate significantly increased, although not all changes were clinically significant. Other lipids and HbA1c did not change. Vitamin D deficiency was seen in 32–54% throughout treatment. BMD z-scores after 12 to 24 months of testosterone treatment remained below z-scores before the start of GnRHa treatment. Clinical Implications: Adolescents need to be counseled about side effects with potential longer term implications such as increased hematocrit and decreased HDL cholesterol and decreased BMD z-scores. They should be advised on diet, including adequate calcium and vitamin D intake; physical exercise; and the use of tobacco and alcohol to avoid additional risk factors for cardiovascular disease and osteoporosis. Strengths & Limitations: Strengths are the standardized treatment regimen and extensive set of safety parameters investigated. Limitations are the limited duration of follow-up and lack of a control group so some of the observed changes may be due to normal maturation rather than to treatment. Conclusion: Testosterone effectively induced virilization beginning within 3 months in the majority of adolescents. Acne was a common side effect, but no short-term safety issues were observed. The increased hematocrit, decreased HDL cholesterol, and decreased BMD z-scores are in line with previous studies. Further follow-up studies will need to establish if the observed changes result in adverse outcomes in the long term. Stoffers IE, de Vries MC, Hannema SE. Physical Changes, Laboratory Parameters, and Bone Mineral Density During Testosterone Treatment in Adolescents with Gender Dysphoria. J Sex Med 2019;16:1459–1468.