Concerns about the effects of gender-affirming hormonal treatment (HT) on bone mineral density (BMD) in transgender people exist, particularly regarding the decrease in estrogen concentrations in transmen. Although it is known that HT is safe for BMD in the short term, long-term follow-up studies are lacking. Therefore this study aimed to investigate the change in BMD during the first 10 years of HT, to determine whether HT is safe and if assessing BMD during HT is necessary. A follow-up study was performed in adult transgender people receiving HT at the VU University Medical Center Amsterdam between 1998 and 2016. People were included if they were HT naive and had a dual-energy X-ray absorptiometry (DXA) scan at the start of HT. Follow-up DXA scans performed after 2, 5, and/or 10 years of HT were used for analyses. The course of BMD of the lumbar spine during the first 10 years of HT was analyzed using multilevel analyses. A total of 711 transwomen (median age 35 years; IQR, 26 to 46 years) and 543 transmen (median age 25 years; IQR, 21 to 34 years) were included. Prior to the start of HT, 21.9% of transwomen and 4.3% of transmen had low BMD for age (Z-score < -2.0). In transwomen lumbar spine BMD did not change (+0.006; 95% CI, -0.005 to +0.017), but lumbar spine Z-score increased by +0.22 (95% CI, +0.12 to +0.32) after 10 years of HT. Also in transmen lumbar spine BMD did not change (+0.008; 95% CI, -0.004 to +0.019), but lumbar spine Z-score increased by +0.34 (95% CI, +0.23 to +0.45) after 10 years of HT. This study showed that HT does not have negative effects on BMD, indicating that regularly assessing BMD during HT is not necessary. However, a high percentage of low BMD was found prior to HT, especially in transwomen. Therefore, evaluation of BMD before start of HT may be considered. © 2018 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals, Inc.