There has been a recent rise in the number of people who hold a non-binary gender identity. However, the proportion of young people attending gender services who identify as non-binary has not yet been investigated. This article presents the findings from a pilot study of newly designed questionnaire, the Gender Diversity Questionnaire, which included questions about gender identity and gender expression. Responses from 251 adolescents attending the United Kingdom’s National Gender Identity Development Service between June 2016 and February 2017 are reported here. The majority, 56.9%, of young people identified as trans, 29.3% identified as a binary gender (male or female), 11% identified as non-binary and 1.2% as agender. There were no significant differences in self-defined identities based on assigned gender or age. However, once young people were separated into these groups, some of them were very small; thus, a larger sample is required. In terms of aspects of gender expression that were important to the young people, the data formed five themes – name and pronouns, external appearance, the body, intrinsic factors and ‘other’. Strengths and weaknesses of the research are discussed as well as future work that will be conducted.