An increasing number of transgender children and adolescents opt to receive gender-affirming hormone therapy (GAHT) during puberty in order to develop pubertal characteristics aligned with their identified gender, which differs from assigned sex at birth. Despite evidence suggesting that hormonal changes from childhood to late adolescence influence brain development, the effects of sex steroids on brain development during puberty remain unclear. In an ongoing longitudinal magnetic resonance imaging (MRI) study we aim to investigate the effects of puberty and sex steroids on structural and functional brain development. Early adolescent transgender youth undergoing GAHT and cisgender controls are recruited at an early stage of puberty (month 1, Tanner stage 2), and invited back at the midpoint (month 15) and after completion of puberty (month 30). At each time point we collect MRI data, including diffusion-weighted imaging (DWI). In addition to demographic and neurocognitive measures, the Children's Depression Inventory (CDI-2), Strengths and Difficulties Questionnaire (SDQ), Multidimensional Anxiety Scale for Children (MASC 2), Gender Identity Questionnaire, and Body Image Questionnaire are used to assess mental health. We analyzed MRI and questionnaire data from the study’s first time point (n=25, 13 transgender youth, mean age=13.7+/-1.04; 12 cisgender youth, mean age=11.8+/-0.97). To examine structural brain connectivity, we performed tractography-based analysis of DWI data using pyAFQ to delineate major white matter tracts. We focused on tissue properties of the corpus callosum, superior longitudinal fasciculi, and thalamic radiations because they have been previously found to be implicated in sex differences and mental health problems, such as depression and anxiety. Preliminary results from the study’s first time point show no significant group differences of mean diffusivity and fractional anisotropy in the white matter tracts of interest between the gender identity and natal sex groups. We also found that fractional anisotropy of the anterior forceps correlated negatively with CDI-2 (r=-0.43, p=0.039) and SDQ scores (r=-0.52, p=0.01). New evidence concerning the role of sex steroids in structural brain development will provide critical insights into the effects of GAHT in transgender youth and could have implications for children’s health, in adding to our knowledge of the gender-specific and sex-specific development of psychopathology in puberty.