Hormonal interventions in adolescents with gender dysphoria may have adverse effects such as reduced bone mineral accrual.To describe bone mass development in adolescents with gender dysphoria treated with gonadotropin-releasing hormone analogues (GnRHa), subsequently combined with gender affirming hormones.Observational prospective study..51 transgirls and 70 transboys receiving GnRHa and 36 transgirls and 42 transboys receiving GnRHa and gender affirming hormones, subdivided in early and late pubertal groups.bone mineral apparent density (BMAD), age and sex specific BMAD Z-scores, and serum bone markers.At start of GnRHa treatment, mean areal bone mineral density (aBMD) and BMAD values were within the normal range in all groups. In transgirls the mean Z-scores were well below the population mean. During two years of GnRHa treatment BMAD stabilized or showed a small decrease, whereas Z-scores decreased in all groups. During three years of combined administration of GnRHa and gender affirming hormones, a significant increase of BMAD was found. Z-scores normalised in transboys but remained below zero in transgirls. In transgirls and early pubertal transboys all bone markers decreased during GnRHa treatment.BMAD Z-scores decreased during GnRHa treatment and increased during gender affirming hormone treatment. Transboys had normal Z-scores at baseline and at the end of the study. However, transgirls had relatively low Z-scores, both at baseline and after three years of estrogen treatment. If this results in adverse outcomes such as increased fracture risk in transgirls as they grow older is currently unclear.