For transgender and other minority populations, the first step in reducing health disparities must be to identify and describe the issues facing them. However, it can be challenging to identify transgender beneficiaries based on administrative data alone. We aim to better understand how the change from the International Statistical Classification of Diseases (ICD)-9-CM diagnosis codes to ICD-10 diagnosis codes in October 2015 has affected our ability to identify transgender patients, and whether different populations of beneficiaries are identified using the two strategies. Our initial findings indicate that while there is overlap between the population of transgender beneficiaries identified using ICD-9 codes and those identified using ICD-10 codes, there may be large numbers of transgender beneficiaries 'hidden' by the exclusive use of one set of diagnosis codes. In this session, our findings will be discussed, as well as further work undertaken to characterize those transgender beneficiaries who are only identified with ICD-9 codes, those who are only identified by ICD-10 codes, and those who are identified by both sets of codes.