Estrogen deficiency is classically associated with bone loss in both men and women. In transgender women, after being submitted to gender-affirming surgery (GAS), the main goal of hormone therapy (HT) is to maintain the female phenotype and prevent the consequences of the orchiectomy-related hypogonadal state. The aim of this study was to evaluate the impact of GAS on bone mass in transgender women. A total of 142 trans women attending the outpatient Gender Identity Program were sequentially enrolled. Patients aged < 20 and > 60 years (n=15), with gluteal silicone prosthesis (n=26) and without FSH dosage after surgery (n=9) were excluded. Anthropometric evaluation, laboratory tests and dual-energy X-ray absorptiometry (DXA) were performed in all patients during the follow-up. In women undergoing CAS (CAS-Y), DXA was performed at least 12 months after surgery and with estrogen therapy. In the other women (CAS-N), tests were performed after at least 3 months of standardized treatment (estradiol plus spironolactone or cyproterone acetate). Patients with testosterone values still above the reference for women were not excluded as long as they were on regular HT. Ninety two trans women were included. Among them, 30 had performed CAS, and had DXA assessment performed 37 months (21-78) after surgery. The mean age and BMI were 37 years (33 - 46) and 24.9 kg/m² (23.1 - 27.5) in patients CAS - Y and 30 years (24 - 36) and 24.3 kg/m² (21.5 - 28.5) in patients CAS - N. Trans women CAS-Y were significantly older (p=0.000). No difference was observed regarding estradiol levels between the groups [105.7pmol/L (48.4-207.8) and 147.5 pmol/L (71.9-284.5), p=0.622]. Free androgen index (FAI) was significantly higher [0.45 (0.17 - 1.63) and 4.47 (0.70 - 36.4), p=0.002] and FSH significantly lower [60.4mIU/ml (37.9 - 75.6) and 2.6mIU/ml (0.6 - 4.4), p=0.000] in trans women CAS - N. BMD (g/cm²) and Z-score of lumbar spine, femoral neck and total femur did not differ significantly between the groups. Considering all participants, the lumbar spine BMD was negatively correlated with FSH levels (r=-0.343, p=0.005), which remained significant even after adjustments for FAI. When only CAS - Y trans women were considered, a negative correlation was found between FSH levels and lumbar spine (r=-0.598, p=0.001) and hip (r=-0.404, p=0.033) BMD. In a multiple regression model adjusted for age and surgery, women with FSH > 35 mIU/ml presented a prevalence rate ratio of 11.79 for low bone mass (p=0.040, IC 95% 1.19 - 124.39). The results of this pilot study in trans women show no difference in bone mass according to GAS status. However, long-term elevated FSH levels observed in some post GAS - trans women, even on HT, presented a negative association with bone mass. Further studies with greater sample sizes are needed to confirm the impact of GAS on bone mass and fracture risk.