The Trans20 study aims to prospectively document the natural history of gender diversity in trans and gender diverse children and adolescents from Victoria, Australia, and to investigate long-term outcomes (gender, mental health, physical health, schooling, family functioning, and quality of life) for those receiving gender affirming interventions. Participants include patients aged between 3 and 17 years old who attended the Royal Children's Hospital Gender Service (RCHGS) in Melbourne, Australia, between February 2017 and February 2020. Data from the first 2 years of the study indicate that on the day of the first appointment with the RCHGS, the large majority of patients (75.3%) were aged 12 years or older, 18.6% were aged between 6 and 11 years, and 6.1% were aged 5 years or younger.