61 Assessing medical decision-making competence in transgender minors 4 INTRODUCTION In December 2020, the High Court of Justice in London ruled that, in the United Kingdom, transgender minors aged ≤15 years are highly unlikely to fully understand the long-term effect of puberty suppression (PS; using gonadotropin-releasing hormone analogues (GnRHa)) and to give informed consent (IC) (Dyer, 2020b). Other countries and states have considered or applied similar age-based restrictions in access to this care as well (Walch et al., 2021). However, evidence regarding transgender minors’ medical decision-making competence (MDC) was lacking until now. To our knowledge, the current study is the first to present empirical outcomes of assessment of transgender minors’ MDC. Transgender people have a feeling of discrepancy between their birth-assigned sex and gender identity (World Health Organization, 2022). In this article, the term ‘(birth-assigned) sex’ is used for an anatomic or chromosomal determination, as opposed to gender, which refers to an internal sense of self as man, woman, another gender or no gender. When puberty starts, transgender minors have to deal with body changes they abhor. In the 1990s, the Dutch introduced treatment with PS, which allows transgender adolescents to further mature and accrue life experience before decisions are made regarding successive gender-affirming medical treatment (GAMT) with permanent physical changes (CohenKettenis, Steensma, & de Vries, 2011; Delemarre-Van De Waal & Cohen-Kettenis, 2006; Hembree et al., 2017). In the Netherlands, transgender adolescents undergo a diagnostic trajectory, including a psycho-diagnostic assessment and several monthly sessions with a mental health provider over a longer period of time (usually ~6 months), when assessing eligibility for PS. PS at early stages of puberty improves psychological functioning and ameliorates general functioning, and physical outcome may be better (Anacker et al., 2021; de Vries et al., 2011a; van der Miesen, Steensma, de Vries, Bos, & Popma, 2020). As far as currently known, the effects of this treatment are fully reversible when discontinued (Hembree et al., 2017). However, there are worries about the impact of PS on physical, cognitive, and psychosocial development and the capability of making decisions about this treatment with profound implications (e.g., regarding fertility) at this young age (Anacker et al., 2021; Chen et al., 2020; Kreukels & Cohen-Kettenis, 2011). Minors’ MDC for interventions is a major issue in paediatric ethics. Therefore, according to the international guidelines, one of the criteria for transgender adolescents to start PS is having sufficient mental capacity to give IC (Coleman et al., 2012; Hembree et al., 2017). Of note, GnRHa are standard of care for treatment for children with precocious puberty (Carel et al., 2009). Minors are a protected population and, in most circumstances, not accorded the legal right to consent. Local jurisdictions determine age limits for minors’ alleged MDC, which vary