INTRODUCTION AND OBJECTIVE: As increasing numbers of adolescents with gender dysphoria(GD) start gonadotrophin-releasing hormone agonists (GnRHa), their unknown affects make fertility preservation (FP) increasingly important and strongly recommended, prior to starting hormone treatment (HT). We investigated the take up, hormonal profile and sperm quality in adolescents undergoing FP via cryopreservation or surgical sperm retrieval (SSR), prior to starting HT.METHODS: Data for adolescents wanting to FP was prospectively maintained. Young people were counselled for FP and serum testosterone (T), FSH, and LH were recorded prior to providing semen samples. Semen samples were classified by concentration; normal >15 mil/ml, oligospermia <15 mil/ml, poor <1 sperm/slide, azoospermia. If the sample was insufficient or the patient was unwilling to masturbate, SSR was offered using electroejaculation (EE)AETESE.RESULTS: Overall, 122 patients were referred for FP (median age 16.1 years [12.4-18.7]). 23 patients (19%) declined FP. 64% (n [78, median age 16.7) successfully masturbated and cryopreserved an average of 6.6 straws. In this subcohort, mean total sperm count was 34.24 million. Sperm concentration was normal in 44%, 36% had oligospermia, 9% had poor sperm concentration, and 12% were azoospermic. Mean blood results showed a T of 12.32 nmol/l [1.2-28.4], FSH 3.83 IU/L [0.8-21.1], LH 4.39 IU/L [0.8-16.7]. SSR was required in 24 patients. These patients were younger, with a median age 14.8 [12][13][14][15][16][17][18]. EE was successful in 4 (16%), while the rest proceeded with testicular sperm extraction; 4 mTESE patients were azoospermic. Of the remaining 16 patients, an average of 5 vials were stored. Semen parameters in this cohort were poor -but possibly adequate for ICSI. Hormone levels were similar to the masturbation cohort. Mean T 11.7nmol/l [5.9-12-8], FSH 3.4 IU/L [1.2-4.9], LH 3.5 IU/L [2. 3-4.7]. SSR resulted in no serious complications.3 patients had previously used GnRHa pre SSR. Following a washout period, 1 remained azoospermic despite a T of 21 nmol/l. The other 2 had sperm found with abnormal morphology and motility.CONCLUSIONS: This the largest cohort of transgender girls referred for FP in the UK. Semen concentrations were below 15 mil/ml in 67% of those who masturbated, but an average of 6.6 straws were frozen. SSR wasn't successful in all despite T levels >8nmol/l. Testicular sperm parameters were poor in all cases. Previous GnRHa use may have an adverse effect though numbers are small. Masturbation is often possible but SSR provides patients with a safe alternative to potentially have their own biological children.