Background: Gender affirming hormonal treatment (GAHT) is a cornerstone in transgender care. National data are sparse regarding use of hormonal treatment by transgender persons. Aim: To assess use of GAHT in transgender persons. Design: National register-based cohort study in Danish transgender persons followed from 2000 until 2018. The main outcome measure was prescription and purchase of GAHT. Persons with ICD-10 diagnosis code of “gender identity disorder” (CGI-cohort) and persons with legal sex change but without diagnosis (CPR-cohort) were included. In the CGI-cohort, transgender women were defined by prescription of estrogen and/or cyproterone acetate and/or testosterone-5-alpha reductase inhibitors, and transgender men were defined by prescription of testosterone after study inclusion. Discontinuation of GAHT was defined as no purchase of GAHT ≥13 months or shift from feminizing to masculinizing hormone treatment, or vice versa. Results: The cohort included 2789 transgender persons (n = 1717, CGI-cohort and n = 1072, CPR-cohort). The median age (interquartile range) at study inclusion was 26.1 (17.7) years for persons assigned male at birth (n = 1447) and 22.5 (10.5) years for persons assigned female at birth (n = 1342). In the CGI-cohort, the event rate for GAHT in transgender women increased from 4.0 (95% confidence interval [CI]: [3.1; 5.2]) events per 100 person in year 2000–2005 to 20.6 (17.8; 23.7) between 2014 and 2018. In transgender men, the event rate of GAHT increased from 4.2 (2.8; 6.2) to 18.8 (16.4; 21.6). The rate of discontinuation of GAHT was 0.06 (95% CI 0.049; 0.071) per person year. Conclusions: The event rate of GAHT increased during 2000–2018. Our data suggested high adherence to GAHT.