© 2018 International Society for Sexual Medicine Introduction: The prevalence of gender dysphoria in children is not known; however, there are some data on the sex ratio of children referred to specialized gender identity clinics. Aim: We sought to examine the sex ratio of children, and some associated factors (age at referral and year of referral), referred to the Gender Identity Development Service in the United Kingdom, the largest such clinic in the world. Methods: The sex ratio of children (N = 1,215) referred to the Gender Identity Development Service between 2000–2017 was examined, along with year of referral, age-related patterns, and age at referral. Main Outcome Measure: Sex ratio of birth-assigned boys vs birth-assigned girls. Results: The sex ratio significantly favored birth-assigned boys over birth-assigned girls (1.27:1), but there were also age and year of referral effects. The sex ratio favored birth-assigned boys at younger ages (3–9 years), but favored birth-assigned girls at older ages (10–12 years). The percentage of referred birth-assigned boys significantly decreased when 2 cohorts were compared (2000–2006 vs 2007–2017). On average, birth-assigned boys were referred at a younger age than birth-assigned girls. Clinical Implications: The evidence for a change in the sex ratio of children referred for gender dysphoria, particularly in recent years, matches a similar change in the sex ratio of adolescents referred for gender dysphoria. The reasons for this remain understudied. Strength & Limitations: The United Kingdom data showed both similarities and differences when compared to data from 2 other gender identity clinics for children (Toronto, Ontario, Canada, and Amsterdam, The Netherlands). Such data need to be studied in more gender identity clinics for children, perhaps with the establishment of an international registry. Conclusion: Further study of the correlates of the sex ratio for children referred for gender dysphoria will be useful in clinical care and management. de Graaf NM, Carmichael P, Steensma TD, et al. Evidence for a Change in the Sex Ratio of Children Referred for Gender Dysphoria: Data From the Gender Identity Development Service in London (2000–2017). J Sex Med 2018;15:1381–1383.